Marine Corrosion And Biodegradation

Introduction

„How inappropriate to call this planet Earth when it is quite clearly Ocean.” (Arthur C. Clarke). Water covers about seventy percent of Earth’s surface and oceans represent ,,source of life”. Oceans influenced over time people's lives and serve as an important source of sustenance. [] .

For a long time the oil industry was the most important of Romania’s industrial sector. The year 1857 marked the beginning of the first petroleum production. Romania was the first country in the world with a petroleum production officially registered in the international statistics, certified by The Science of Petroleum in 1938. It was followed by the United States in 1859, Italy in 1860, Canada in 1862 and Russia in 1863.

„The interest for off-shore drilling in the continental platform of the Black Sea was materialized for the first time in 1967, when the Romanian Government approved two major projects: one for geological prospect and the second one for off-shore drilling. The first jack-up rig "Gloria" was built in 1976. The project was partially financed by an American offshore company and was under the close technical supervision of the American Shipping Bureau. The jack-up was designed to drill up to 90m deep. .” []

In this project I want to present the ocean environment impact on offshore structures. Each main parameter of the ocean enivronment will be presented very detalied because I will use all the data needed to build a map of The Black Sea environment and the influence of this on offshore structures. We know that recently it has been discovered important oil resources in the Black Sea and I hope this project will help me in my vocational education.

Chapter 1. The ocean environment

The atmosphere and the oceans are the keys of the ocean environment and they are in constant motion. Together represent the most dynamic component of the ocean. “ Interactions between the ocean and the atmosphere proceed in both directions. They also operate at different rates. Some interesting lag effects, which are of value in long-range weather forecasting, arise through the considerably slower circulation of the ocean.

Thus, enhanced strength of the easterly trade winds over low latitudes of the Atlantic north and south of the Equator impels more water toward the Caribbean and the Gulf of Mexico, producing a stronger flow and greater warmth in the Gulf Stream approximately six months later. Anomalies in the position of the Gulf Stream–Labrador Current boundary, which produce a greater or lesser extent of warm water near the Grand Banks, so affect the energy supply to the atmosphere and the development and steering of weather systems from that region that they are associated with rather persistent anomalies of weather pattern over the British Isles and northern Europe.

Anomalies in the equatorial Pacific and in the northern limit of the Kuroshio Current (also called the Japan Current) seem to have effects on a similar scale. Indeed, through their influence on the latitude of the jet stream and the wavelength (that is, the spacing of cold trough and warm ridge regions) in the upper westerlies, these ocean anomalies exercise an influence over the atmospheric circulation that spreads to all parts of the hemisphere.” []

1.1 Winds and currents

„Wind generated currents are complex and the speed and direction of these currents may be misinterpreted by anglers. Winds generally create only weak currents near the surface that move in the same general direction as the wind only until it gets near a shoreline. A strong prevailing wind blowing for many days in the same direction may create a deeper flow.” []

Fig 1.1 Winds map

Sursa: ( https://earth.nullschool.net/ )

Fig 1.2 Ocean Currents map

Sursa: ( https://earth.nullschool.net/ )

1.1.1 Ocean wind

Ocean wind is defined as the motion of the atmosphere relative to the surface of the ocean. Typically ocean winds are measured very close to the ocean surface by buoys, platforms, and ships. The most common reference height for near-surface ocean wind measurements is 10 meters above sea level. More recently, the advancement of satellite remote sensing has enabled high-resolution near-surface ocean wind measurements from space using both passive and active instruments. Today, the combination of all available satellite wind measurements can provide global coverage over the ice-free oceans at multiple times per day.

Fig 1.3 Ocean Winds map

Sursa: ( https://earth.nullschool.net/ )

Ocean wind is measured using either in situ (i.e., on site) or remote sensing (i.e., from a distance) instruments and techniques. In situ wind measurements may come from buoys, ships, or platforms. The most common instrument used for in situ wind measurements is the mechanical anemometer, which utilizes the wind’s resistance to propel a very small turbine to determine the wind speed; these anemometers also have a wind vane, which looks similar to the tail fin of an airplane, which helps the anemometer to always point into the direction of the wind, thus allowing the anemometer to measure both wind speed and direction.

Wind can also be measured remotely using both ground-based and airborne instruments. Ground-based Doppler radar can measure ocean wind using the inbound and outbound radial velocities of hydro meteors from storms within close proximity to the radar station; the range is typically limited to several hundred kilometers due to signal attenuation.

Airborne ocean wind measurements can take place using both active and passive microwave instruments; the microwave frequency band is preferred due to its ability to penetrate through clouds and precipitation and its sensitivity to the ocean surface roughness. The ocean surface responds quickly to the motion of the air above, which provides a distinct roughness pattern depending on the relative speed and direction of the wind with respect to the ocean surface. The roughness of the ocean surface provides a specific “brightness” which can only be here observed using passive microwave radiometers; with the right combination of specific microwave wavelengths and processing algorithms, the brightness of the ocean surface can be accurately translated to a near-surface wind speed.

Specific microwave wavelengths are sensitive to a feature known as Bragg scattering, which is a characteristic of centimeter-scale ocean surface waves known as capillary waves. Capillary waves are directly influenced by changes in near-surface winds, which enable specially tuned airborne radars to observe these changes. These airborne radars transmit microwave pulses of energy to the ocean surface, which immediately scatters a portion of the reflected energy back to the radar. Once the radar cross section is normalized, the near-surface wind speed can be computed as a function of the backscattered energy. In contrast to passive microwave radiometers, the active radar system can combine measurements from different azimuth angles to derive the approximate direction of the wind. Due to the dependence on the principal of Bragg scattering, these types of radars are specifically categorized as scatterometers.

The satellite remote sensing of wind speed and direction (i.e., vectors) over the ocean began with the first satellite microwave scatterometer aboard SKYLAB (May 25, 1973 – June 22, 1973), which measured the reflected backscattered energy from the ocean surface.

These unprecedented measurements from space provided the foundation for scientists to develop a more practical understanding of the relationship between ocean surface backscatter, brightness temperatures, and near-surface wind speed. []

SKYLAB led missions: Seasat, Nimbus-7, SSM/I, ERS (ESA), NSCAT, SeaWinds on QuikSCAT, SeaWinds on ADEOS-2, and ASCAT (ESA/EUMETSAT).

The operations of Seasat, ocean-based measurements were the steps which led to the first geophysical model function (GMF) to convert both active and passive microwave measurements into ocean surface wind observations.

Fig 1.4 Estimated Sea Floor Topography from Satellite Altimetry

Sursa: ( https://www.ngdc.noaa.gov/mgg/image/global_grav_large.gif )

1.1.2 Currents and Surface ocean currents

The Earth rotation and the Coriolis Effect influence winds circulation and their impact on the ocean water surface. Coriolis Effect. Suppose our Earth did not rotate and remained stationary, then the athosphere would circulate between the poles and the equator. The Earth rotation produce the deflectation of athmospheric circulation, named Coriolis effect.

Fig 1.5 Coriolis Effect

Sursa: ( http://oceanservice.noaa.gov/education/kits/currents/05currents1.html )

Surface ocean currents form large system of rotating ocean currents, circular patterns called gyres. Gyres flow clockwise in Northern Hemisphere oceans and counterclockwise in Southern Hemisphere oceans and they are caused by Coriolis effect . Near the Earth’s poles, gyres tend to flow in the opposite direction. The Five major gyres are Indian Ocean Gyre, North Atlantic Gyre, North Pacific Gyre, South Atlantic Gyre and South Pacific Gyre.

Fig 1.6 Gyres Map

Sursa: ( http://www.cbc.ca/news2/interactives/ocean-garbage/gfx/five-gyres-map-940d.png )

We know that surface ocean currents flow in a regular pattern, but they are different, they can be: deep and narrow or shallow and wide. Currents are often affected by the shape of the ocean floor. Some move quickly while others move more slowly. A current can also change somewhat in depth and speed over time.

Surface ocean currents carry heat from place to place in the Earth system. This affects regional climates. The Sun warms water at the equator more than it does at the high latitude polar regions. The heat travels in surface currents to higher latitudes. A current that brings warmth into a high latitude region will make that region’s climate less chilly.

Surface ocean currents can create eddies, swirling loops of water, as they flow. Surface ocean currents can also affect upwelling in many places. They are important for sailors planning routes through the ocean. Currents are also important for marine life because they transport creatures around the world and affect the water temperature in ecosystems.” []

Fig 1.7 Currents are cohesive streams of seawater that circulate through the ocean. Some are short-lived and small, while others are vast flows. There are two distinct current systems in the ocean: surface circulation(which stirs a relatively thin upper layer of the sea) and deep circulation (which sweeps along the deep-sea floor)

Sursa: (http://oceanexplorer.noaa.gov/facts/currents.html )

1.1.3 Winds and Currents of The Black Sea

The Black Sea is a deep with a depth to 2212 m elongated basin situated between . Its maximum zonal length is 1175 km. The Crimea peninsula and the Anatolian coast convexity divide the sea into two sub-basins. The minimum width of the sea is 258 km. The broad north-western shelf (NWS) occupies the northwestern part of the sea. Typical width of the shelf along the other coastlines is 2-12 km. Profiles of density show a well-pronounced permanent pycnocline situated at a depth of 150-300m.

The Black Sea is connected with the ocean by the narrow Bosphorus Straits. Density stratification is determined mainly by salinity, which is near 22.5 ppt in the deep-sea against 18-18.5 ppt on the surface. Sea surface temperature (SST) varies seasonally from 8°C to 30°C, while the deep-sea temperature is about 8.5°C. Winter cooling on the NWS could reduce SST up to 6°C and produces a distinctive feature of the Black Sea thermal stratification, the so-called cold intermediate layer (CIL) situated at a depth of about 50-90m.

A permanent feature of the upper layer circulation is the Rim Current, encircling the entire Black Sea and forming a large-scale cyclonic gyre. Direct observations of the current velocity from surface buoys suggest that the maximum speed of this stream is usually 40-50cm/s increasing sometimes up to 80-100cm/s. The Rim Current is concentrated above the shallow pycnocline and the volume transport of the current is estimated to be 3-4 Sv. There are also two smaller cyclonic gyres in the western and the eastern parts of the basin. Cyclonic circulation induces the rise of the sea level toward the coast. The amplitude of sea level variation in space depends on season and ranges from 25 to 40 cm.

Maps of surface currents obtained from altimetry manifest an obvious annual cycle of the circulation. The Rim Current is the most intense in winter-spring seasons. Winter circulation shows two gyres but in spring the current jet belts the whole basin along the bottom slope. Summer circulation attenuates significantly and in the autumn season the Rim Current usually breaks on the set of eddies. Seasonal variability of currents is well presented by the temporal evolution of potential and kinetic energy and has a simple physical explanation. Increase of the cyclonic vorticity of wind in January produces intensification of the upwelling on the bottom of the Ekman layer and rise of the pycnocline. The most shallow pycnocline observes a quarter of period (i.e. three months) after the most intense wind stress curl. However the rise of pycnolcline in the central part of the basin should be compensated by its deepening near the coast due to the conservation of the fluid volume above the pycnocline. The deepening of pycnocline near the coast is significantly larger than its rise in the open sea as the volume of fluid replaced in the center of the basin should be preserved along the beach. Therefore the slope of pycnocline toward the coast increases significantly at the beginning of spring. The intensity of the Rim current, which is in geostrophic balance, is highest at the same time. The weakening of the wind stress curl vice versa is accompanied by the deepening of pycnocline in the open sea and its rise near the coast. Thus, the phase of current oscillation is determined by the seasonal cycle of the wind stress curl.

Conclusions: In winter, the basic flow – the Rim Current, spanning the entire the Black sea, is stable and intense. At the end of Spring, the Rim Current weakens and begins meandered. Vigorous mesoscale eddies, meanders and filaments could be revealed which pinch off coastal recirculation regions or Rim Current intrusions towards deep water. Several anticyclonic eddies have been observed, located between the Rim Current and the coast: along the Crimean Peninsula and close to the Bosphorus region.

The most prominent sub-basin scale feature is perhaps the quasi-permanent Batumi eddy located in the easternmost basin of the Black Sea. These eddies play an important role in the transport of nutrients from coastal areas to deepwater ones.

Fig 1.8 Rim Current in the Black Sea – Feb 2012

Sursa: ( http://marine.copernicus.eu/web/18-multimedia.php?item=757 )

The most intense currents are observed approximately three months later after intensification of the wind stress curl. Accordingly, the weakest currents are observed approximately three months after amplification of the wind stress curl. The decomposition of the observed sea level to the empirical orthogonal functions (EOF) is carried out to prove that the basin-averaged wind stress curl is responsible for the annual variability of the Black Sea currents.

The Black Sea circulation also manifests significant inter-annual variability. Inter-annual variability is obvious in both potential and kinetic energy evolution. Annual mean energy as well as the amplitude of its seasonal variation is changed a few times from one year to another. Inter-annual variability is also observed in the structure of eastern and western gyres. The energy budget equation is considered to understand the nature of inter-annual variability of the Black Sea circulation. This equation states that the rate of change of basin-integrated energy is equal to the difference between the work exerted by the wind stress over the basin and dissipation due to horizontal friction in the system. It is sufficient to have the altimeter and the wind stress data to estimate all terms of the energy budget equation. It is found that temporal variations of the left and right hand sides of the equation are in phase, implying that the basin integrated wind stress work is the main source of total energy variations over the basin.

http://www.iasonnet.gr/past_conf/abstracts/korotaev.html

Fig 1.9 Winds in the Black Sea – July 2016

Sursa: ( https://www.meteoblue.com/en/weather/map/wind/black-sea )

Fig 1.10 Winds in the Black Sea – July 2016

Sursa: ( https://earth.nullschool.net/ )

Fig 1.11 Winds in the Black Sea – July 2016

Sursa: ( https://earth.nullschool.net/ )

Fig 1.12 Winds in the Black Sea – July 2016

Sursa: ( https://earth.nullschool.net/ )

1.2 Waves

Fig 1.13 Waves Map

Sursa: ( https://earth.nullschool.net/ )

1.2.1 Surface gravity waves

Gravity waves on the surface of water are one of the most visible manifestations

of fluid motion, observed as disturbances to the free surface in large bodies of water. “The process at work is relatively easy to comprehend: A fluctuation causes water

to rise above the equilibrium surface level, gravity pulls it back down because water

is heavier than air, inertia acquired during the falling movement causes the water

to penetrate below its level of equilibrium, and a bouncing motion results. The

oscillation is similar to that of a spring that has been stretched and released. The

‘spring’ action in a surface water wave is gravity, hence the name of surface gravity

wave.

Water motion under a surface wave is very nearly oscillatory, with almost no net

displacement. Thus, surface waves, like most other fluid waves, are a mechanism

by which the fluid moves energy from one area to another without involving any

significant movement of the fluid itself. Energy and information are carried with the

fluid acting as the support medium rather than as the messenger. That property

has a fundamental implication: Surface waves by their very nature are unable to

transport any mass, including dissolved pollutants and suspended matter.

The energy carried by surface waves, however, must eventually be dissipated

somewhere and will affect the water contents there. For example, wave energy

can be converted into turbulent mixing under wave breaking, and the resulting

mixing can stir the local water contents, such as pollutants, biological matter and

heat. Wave energy can also be dissipated by bottom friction under wave-induced

oscillatory flow, and this friction can in turn create a shear stress sufficiently strong

to entrain sediments into suspension. In sum, waves do not contribute directly to

transport and redistribution of fluid-borne elements along their travel but can be

effective means by which a remote source of energy can affect the concentration of

dissolved and suspended matter at a distant location. This remark holds true for

most types of waves.” []

1.2.2 Breaking waves in shallow water

The effect of change of wave height when surface waves enter shallower water is named wave shoaling. represent the physical limit to the steepness of the water waves, so the wave breaks and dissipates its energy when this limit is exceeded.

Mathematical model

Fig 1.14 Regular Wave

Sursa:

Assignments:; ; ; ; ; ; .

When the wave breaks, their forms are very different because the breaker type is a function of the wave steepness, , and the seabed slope, .

Then the breaker types dependence on the Iribarren number:

Table 1.1: The Breaker Types

Fig 1.15 Breaking Waves; a. Breaker Types; b. Drawings based on illustrations by Charles Pilkey

Sursa:(https://en.wikipedia.org/wiki/Iribarren_number; http://www.indyweek.com/indyweek/a-closer-read-of-the-north-carolina-shore/Content?oid=1195080 ; http://ukka.co/pics/the-wave/; http://stockpix.com/stock/nature/408902.htm; https://pt.pinterest.com/pin/237564949070903363/ )

1.2.3 Wave slamming forces on vertical cylinders

A frequently occurring event that have an important impact on dynamics of offshore structures is slamming wave impact on cylindrical structures. This affect pipelines of vessels, cylindrical structural members of offshore platforms and monopiles of offshore wind turbines. Waves impact can be very violent that they cause damage to cylindrical members of offshore structures.

We need to have to have precise information about the maximum loads which could occur on these structures during their lifetime. “Hereby, a distinction has to be made between the impact on a rigid and deformable body. The loads acting on a flexible structure are smaller than these acting on a rigid one, since a part of the impact energy will be absorbed by the deformation. However, the degree of this decrease with respect to rigid bodies is not known so far.

Various analytical studies already exist and they describe water impact and deformation of cylindrical objects.” []

Many offshore wind turbines are supported by mono-piles. In this case, the wave load is generally predicted by using the Morison equation.

Another model used for the impact force by considering the breaking wave as a vertical wall of was proposed by Goda in the year 1966.

Fig 1.16 Definition Sketch

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1.3 Solar and Lunar Ocean Tides

1.3.1 Introduction to Solar and Lunar Ocean Tides

The tide is defined as the periodic rise and fall of the waters of the ocean and its inlets, produced by the attraction of the moon and sun, andoccurring about every 12 hours.The sun is much bigger than the moon and it exert only 0.46 times of the moon’s tidal force since it is much farther away from the earth.

The hight tide, the water reaches its highest level, and the low tide represent the creast and trough of a wave with hundreds of kilometers.

Fig 1.17 Diagram of tidal island at low tide and high tide

Sursa: ( https://en.wikipedia.org/wiki/Tidal_island )

Fig 1.18 Diagram of tidal island at low tide and high tide

Sursa: ( http://i.imgur.com/XjWW1Yw.jpg )

,,The alternating pattern of rising and falling sea level with respect to land is what we know as the tides. What causes this "motion of the ocean"? In one word, gravity. Specifically, the gravitational forces of the Sun and Moon. 
The key to understanding how the tides work is understanding the relationship between the motion of our planet and the Moon and Sun. As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward. 
However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves. The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide. 
Water on the opposite side of Earth facing away from the Moon also bulges outward (high tide), but for a different and interesting reason: in reality, the Moon and the Earth revolve together around a common gravitational center between them, or center of mass. Here's a rough but helpful analogy: picture yourself swinging a heavy object attached to a rope around your body as you rotate. You have to lean back to compensate, which puts the center of mass between you and the object. With the Earth-Moon system, gravity is like a rope that pulls or keeps the two bodies together, and centrifugal force is what keeps them apart. Because the centrifugal force is greater than the Moon's gravitational pull, ocean water on the opposite side of the Earth bulges outward. 
The same forces are at play as the Earth revolves around the Sun. The Sun's gravity pulls ocean water toward the Sun, but at the same time, the centrifugal force of the combined Earth-Sun revolution causes water on the opposite side of Earth to bulge away from the Sun. However, the effect is smaller than the Moon, even given the greater mass of the Sun (greater mass means greater gravitational force). Why? Simply because The Sun is so far away — over 380 times farther away from the Earth than the Moon. 
Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal. 
On the other hand, if the Sun and the Moon are 90 degrees apart in relation to an observer on Earth as during the First Quarter Moon or Third Quarter Moon (sometimes called half moons), then high tides are not as high as they normally would be. This is because despite its greater distance, the Sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon's pull. This phenomenon of lower high tides is called a neap tide. 
The height of the tides can also vary during the course of a month because the Moon is not always the same distance from the Earth. As the Moon's orbit brings it in closer proximity to our planet (closest distance within a moon cycle is called perigee), its gravitational forces can increase by almost 50%, and this stronger force leads to high tides. Likewise, when the Moon is farther away from the Earth (furthest distance is called apogee), the tides are not as spectacular. 
Tides most commonly occur twice a day (diurnal). Tides can also occur as two high waters and two low waters each day (semi-diurnal). However, these periods do not happen at the same time each day. This is because the Moon takes slightly longer than 24 hours to line up again exactly with the same point on the Earth – about 50 minutes more. Therefore, the timing of high tides is staggered throughout the course of a month, with each tide commencing approximately 24 hours and 50 minutes later than the one before it. 
There are many factors involved in predicting the tides. In addition to the motion of the Moon and Sun described above, timing of the tides are also affected by the Moon's declination (angular height above the equator), local geography of the coastline, topography of the ocean floor, and depth of the water, among other considerations. Thus, the tides can't be perfectly predicted solely by astronomical calculations that track the Sun and Moon. For greatest accuracy, tide prediction tables always integrate data from actual observation, often over a period of many years.”[]

Fig 1.19 Distribution of tide-raising forces on earth

Sursa:

Fig 1.20 (a) Uniform and distorted water envelope arount the earth surface, before and after the tidal force acts on the water envelope (b) The change in water level during one full rotation of the earth

Sursa:

1.4 Seismological Considerations, Earthquakes and Seaquakes

Seaquakes are generally considered the phenomena that a ship or a floating structure are shocked by the seismic waves which are propagated through the seawater. The effects of seaquakes on offshore floating structures like the dynamic pressure acting on offshore structures, are studied for many years and both effects are concerned on the compressibility of fluid and the free surface. The resonance is the important role of the effects of seaquakes on offshore floating structures. Offshore floating structures are designed to withstand or resist the effects of earthquakes, but the effects of the vertical oscillations on such offshore floating structures remains unclear.

Seaquakes are defined as a phenomenon in which a ship or a offshore floating structure receive a shock caused by a submarine eruption or earthquake that is transformed in seismic waves which are propagated through the seawater

An earthquake causes a sea-bed deformation such as an upheaval or a subsidence in a very short time. Consequently the disturbances in the seawater propagate all around as long surface waves and grow extremely-developed waves called tsunami.[]

1.5 Ice Environment

In cold areas with ice-covered waters is very important to know ice interactions on offshore structures. The tree important aspects of ice actions are design ice loads from level ice, dynamic ice actions of resonant character, and the actions caused by ridged ice. []

1.6 Distances and Depths

The evolution of the offshore industry has made the possibility to build offshore structures further from the coast, in deeper waters.

An example can be found in the year 2012, when the average water depth of offshore wind farms was 22 m and the average distance to shore was 29km, but now that average water depth and distance to shore increase. []

“Offshore wind farms have moved further from shore and into deeper waters. At the end of 2015, the average water depth of grid-connected wind farms was 27.1 m and the average distance to shore was 43.3 km. This is primarily the result of increased deployment in Germany during 2015, where sites are an average of 52.6 km from shore. By comparison, UK projects were on average 9.4 km from the shoreline. Dutch projects were sited at an average of 31.4 km away from shore.”[]

Fig 1.21 Average water depth and distance to shore of online, under construction and consented wind farms

Sursa:

Chapter 2. Seabed Mechanics

2.1 Ocean Floor

“The oceans of the world are divided in two areas: the benthic zone or seafloor environment and the pelagic zone or water environment.

The continental slope and beyond make up the benthic zone and includes the deepest part of the ocean floor which are made up of sediments consisting of rock particles and organic remain such as calcium carbonate shells of small organisms.

Fig 2.1 Life Zones of the Ocean (Source: Encarta Encyclopedia, 2009)

Sursa:

From the high-tide mark along the shore to the depths of the ocean are found plants and animals of the sea. Plant and animal life in the benthic zone is most abundant in the coastal waters on the continental shelf.

Fig 2.2 The Ocean floor. Most Offshore Drilling for Oil and Natural gas takes place in the Continental Shelves (Source: Encarta Encyclopedia, 2009)

Sursa:

The benthos lives on and depends on the sea bottom and includes benthic fauna like deposit and filter feeders such as barnacles, bryozoans, sponges, mussels, hydroids, pycnogonid sea spiders and stalked crinoids. Plants are found only in the epipelagic zone of the pelagic zone where there is enough light for photosynthesis. Light does not penetrate below the mesopelagic zone. Benthic organisms are good indicators of oil pollution because they live in the sediment for all or most of their lives with limited mobility and differ in their tolerance to amount and types of pollution.” []

2.2 Bearing Capacities of Offshore Platforms

2.2.1 Scour

Local scour is defined as an erosion of seabed material at a single foundation, caused by complex vortex patterns.

Fig 2.3 Cross-section – offshore wind structure

Sursa: (http://www.dredging.org/documents/ceda/downloads/ceda-nl-_iro2010-9-maart-_presentatie_ir_daniel_rudolph_deltares.pdf )

Bridge scour is the removal of sediment such as sand and rocks from around bridge abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.[]

Fig 2.4 Diagram of how scour holes are generated

Sursa:

2.2.2 Mudslides

“Mudslides are fast-moving torrents of mud and rock, which are no longer capable of defying gravity. Prolonged heavy rain or volcanic activity normally cause mudslides and such torrents are among the most destructive forces in nature.” []

Fig 2.5 Nature forces

Sursa: ( http://www.scpr.org/ )

“In regions of unstable soils occasional pipeline failures during periods of severe weather have been attributed to mudslides. The effects of such downslope soil movements on the deflection and resulting stress in a pipeline have been studied in an attempt to reveal methods of routing or design which would reduce the risk of failure.

Results show that the chances of surviving a slide are increased if the pipeline outer diameter is reduced to as small a value as possible, if the pipe wall thickness is increased, and if some slack is available in the line. The chance of survival is greatest if the slide occurs in a direction perpendicular to the pipeline route since the failure mode is primarily one of tension. When the slide crosses the pipeline at other angles of incidence, the chance of survival is significantly lessened.” []

Chapter 3. Marine Corrosion and Biodegradation

3.1 Introduction

“Corrosion is a natural process, which converts a refined metal to a more stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and stopping corrosion.

In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such as oxygen or sulfur. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal, and results in a distinctive orange colouration. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term "degradation" is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

Fig 3.1 Corrosion effects

Sursa: ( http://gorkath.deviantart.com/ )

Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable. “ []

3.2 Corrosion and Microbial Effects on Ocean Environment

3.2.1 Ocean Atmosphere

The transfer rate of most gases between the atmosphere and ocean is controlled by processes just beneath the water surface.

When this region is highly turbulent, gases can be more rapidly transferred toward or away from the surface. The turbulence is in turn controlled by dynamical factors such as wind speed, sea-state, and wave breaking. In addition other effects such as bubbles, surfactants and rain can have a significant influence.

Carbon dioxide (CO2) is a soluble gas which dissolves in the oceans and is taken up by marine plants (phytoplankton). A natural cycle results in which CO2 is absorbed from the atmosphere in some (generally cooler and more biologically active) parts of the ocean and released back to the atmosphere in other (generally warmer and less biologically active) parts.

This natural cycle has been modified through the addition of CO2 to the atmosphere by human activities. Increasing CO2 concentrations in the atmosphere tend to increase the amount dissolved in the surface ocean. Currently about 29 billion (thousand million) tonnes of CO2 are being added to the atmosphere each year due to fossil fuel burning and deforestation and the oceans are removing about 7 billion tonnes. A similar amount is removed due to increases in plant biomass and soil carbon. []

Fig 3.2 Ocean Atmosphere

Sursa: ( http://www.geomar.de/ )

3.2.2 Seawater

Seawater is one of the most important environments to discuss. This may be especially true when it comes to pipelines, which criss-cross the ocean carrying oil and gas worldwide—and which, of course, cause the most problems if they fail.

Three Important Factors in Seawater Corrosion

1. Chloride Concentration
,,The chloride ions in saline water are one of the most aggressive substances in seawater. The chloride concentration in water is usually called "salinity." In seawater, this usually varies between 3.1 and 3.8 weight percent, depending on the solar evaporation rate of water, precipitation and the dilution of water by freshwater and circulation.

The corrosivity of chloride ions in seawater can be explained based on three factors:

1. Chloride ions can react with dissolved ferrous ions to create ferrous chloride according to the following reactions:
Fe Fe2+ + 2 e-
Fe2+ + 2 Cl- FeCl2

The ferrous chloride produced in this reaction can react with dissolved oxygen and produce ferric oxide (Fe2O3) and ferric chloride (FeCl3), which is considered an oxidizing agent that can enhance the general corrosion rate and pitting corrosion. Ferric ions can shift the corrosion potential (Ecorr) to values that are more than Eb (pitting potential or breakdown potential) and thus cause more severe corrosion.

Fig 3.3 Basic membrane cell used in the electrolysis of brine. At the anode (A), chloride (Cl−) is oxidized to chlorine. The ion-selective membrane (B) allows the counterion Na+ to freely flow across, but prevents anions such as hydroxide (OH−) and chloride from diffusing across. At the cathode (C), water is reduced to hydroxide and hydrogen gas.

Sursa: ( https://en.wikipedia.org/wiki/Chloride )

2. In pitting corrosion, chloride ions are called "aggressive anions" that can influence both pit initiation and growth. They can penetrate the passive film and further increase pit initiation risk. Also, chlorides can worsen pit growth through an autocatalytic process. 

It should be noted that stagnant water is necessary for pitting corrosion to occur. In other words, pitting corrosion is unlikely to happen in areas where water is moving and being replaced.

3. Dissolved oxygen is another important factor that can influence the corrosivity of seawater. The concentration of chlorides can influence the solubility of oxygen in seawater. The highest oxygen concentration can be achieved at 3.5 weight percent sodium chloride.

Fig 3.4 The figure above shows how the combination of chloride concentration and dissolved oxygen concentration results in the maximum corrosion rate.” []

Sursa: ( www.corrosionpedia.com )

“Corrosion by sea water, aqueous corrosion, is an electrochemical process, and all metals and alloys in contact with sea water have a specific electrical potential (or corrosion potential) at a specific level of sea water acidity or alkalinity – the pH.    

 This typical diagram shows the regions where the metal will freely corrode; the region of passivation where stable oxide or other films form and the corrosion process is stifled; the region of pitting corrosion where the corrosion potential of the metal exceeds that of its oxide; and the region of immunity where the metal is normally fully safe to use.  More resistant alloys mean less corrosion, metals like gold platinum and tantalum can resist virtually all corrosion, but for marine service the final choice will always be a compromise with cost. 

Most corrosion resistant metals rely on an oxide film to provide protection against corrosion.  If the oxide is tightly adherent, stable and self healing, as on many stainless steels and titanium, then the metal will be highly resistant or immune to corrosion.  If the film is loose, powdery, easily damaged and non self repairing, such as rust on steel, then corrosion will continue unchecked.  Even so, the most stable oxides may be attacked when aggressive concentrations of hydrochloric acid are formed in chloride environments.  

Sea water, by virtue of its chloride content, is a most efficient electrolyte.  The omni-presence of oxygen in marine atmospheres, sea spray and splash zones at the water-line, and sometimes surprisingly at much greater depths, increases the aggressiveness of salt attack.  The differential concentration of oxygen dissolved at the waterline or in a droplet of salt spray creates a cell in which attack is concentrated where the oxygen concentration is lowest.  Crevices which allow ingress of water and chlorides but from which oxygen is excluded rapidly become anodic and acidic and are hidden start points of corrosion.” [] 

3.2.3 Mineral, Mud, Hydrocarbon Products

“The chemical character of groundwater is influenced by the minerals and gases reacting with the water in its relatively slow passage through the rocks and sediments of the Earth’s crust. Many variables cause extensive variation in the quality of groundwater, even in local areas. Generally, groundwater increases in mineral content as it moves along through the pores and fracture openings in rocks. This is why deeper, older waters can be highly mineralized. At some point, the water reaches an equilibrium or balance, which prevents it from dissolving additional substances.” []

“Iron (Fe0) corrosion in anoxic environments (e.g. inside pipelines), a process entailing considerable economic costs, is largely influenced by microorganisms, in particular sulfate-reducing bacteria (SRB). The process is characterized by formation of black crusts and metal pitting.” []

Fig 3.5 Sulphate-reducing Bacteria cill

Sursa: ( http://sore.net.au/ )

Plain Steels
Unprotected, mild steels are not resistant against corrosion in a marine environment. However, they are usually used in marine environments in the form of sheet piles, ship bodies, etc. after using a suitable protection technique, such as cathodic protection or the application of polymeric coatings. For an unprotected steel structure in seawater, the corrosion rate markedly varies depending on its position relative to the ocean, according to the image below.

Fig 3.6 Diagram Relative loss in metal tickness

Sursa: (Copper Development Association Inc.)

At the bottom of the sea (immersion zone), the seawater is stagnant and has the lowest temperature and oxygen concentration. Therefore, in this area, the corrosion rate is expected to be very low in comparison to other zones. In higher levels from the bottom of the sea, there is the tide zone, where materials are exposed to a cyclic wetting-drying process. This cycle repeats every 24 hours and can increase the corrosion rate. According to previous investigations, the corrosion rate of mild steel in this area is around of 100 µm/yr, while this value for the immersion zone is considered less than 50 µm/yr or even near to zero. High temperatures, saturation by oxygen, and the spray or splash of seawater cause the most severe corrosion at a level that is known as "splash zone." The corrosion in the splash zone reaches to 900 µm/yr (for example in the Cook Inlet in Alaska).

It should be mentioned that exactly beneath the splash zone, the corrosion rate is slightly higher than the other parts of the tidal zone. This higher corrosion rate is due to the creation of an oxygen concentration cell. In this cell, the anode is located beneath the splash zone where the O2 partial pressure is low and the cathode is located at the splash zone where the O2 partial pressure is high.

At levels higher than the sea surface, which is called as "marine atmosphere," a thin film of seawater condenses on the metallic surface and can cause atmospheric corrosion. The intensity of wind, salinity of seawater, and temperature are the most important parameters that can influence marine atmosphere corrosion.

Cathodic protection, painting and sheathing are three helpful methods to prevent the corrosion of steel columns and piles in seawater.

Stainless Steels
Stainless steels have a high general corrosion resistance in seawater due to their protective chromium oxide layer. However, since the chloride concentration of seawater is so high, these alloys are susceptible to pitting corrosion in a stagnant seawater environment. For example, stainless steel type 304, which is a commonly used stainless steel, is not safe against the pitting corrosion in seawater. The pitting resistance of stainless steel increases to an acceptable value when 2% molybdenum is added to the chemical composition of stainless steel, which results in stainless steel type 316. Similar to molybdenum, increasing chromium content in stainless steels can enhance pitting resistance in still seawaters.

Fig 3.6 Diagram Relative loss in metal tickness

Sursa: ( http://www.atlanticstainless.com/ )

Copper Alloys
Copper and its alloys (bronze and brass) are usually resistant against general corrosion in seawater. Therefore, diffident kinds of copper alloys are suggested for use in the marine industry. Sometimes, the chemical composition of brass alloys is modified to perform in a marine environment more efficiently: for example, admiralty brass or naval brass alloys, which consist of 1% tin to prevent dezincification; or arsenical brass, which includes very low amounts of arsenic to inhibit dezincification. Aluminum is usually added to brass in order to improve the erosion corrosion resistance of brass alloys in ship impellers. Cupronickels (copper with 10–30% nickel alloys) are widely used for marine applications due to their great resistance against seawater corrosion.

Fig 3.7 Copper corrosion

Sursa: (http://www.flutemcglinchey.com/photos.html )

Concrete
Chlorides can penetrate into concrete through its flaws (pore spaces and cracks) and touch the reinforced steel rods, which are passivated by the highly alkaline environment of concrete. This can cause localized corrosion. Eventually, the concrete breaks down due the internal pressure of rust growth.

Fig 3.8 Concrete corrosion

Sursa: (https://www.mottmac.com/article/884/materials-durability)

Aluminum
The corrosion resistance of aluminum and its alloys in marine environments significantly depends on the alloying elements and surface finish. For example, the presence of iron and/or copper in aluminum decreases the corrosion resistance of aluminum. However, 5xxx series aluminum alloys, which include magnesium, are usually good candidates for use in marine applications (such as 5052 alloy). Moreover, creating a hard anodized layer (thick aluminum oxide layer) on the aluminum surface can inhibit marine corrosion.

Fig 3.9 Aluminum corrosion

Sursa: (http://nautechmarine.co.nz/aluminium-boat-repair/about-corrosion/)

Titanium and Titanium Alloys
Titanium and titanium alloys represent one of the best choices in marine services. Despite its high price, titanium should be considered.

Fig 3.10 Titanium corrosion

Sursa: (http://www.tradeindia.com/fp2094302/Titanium-Tubes-ASTM-B861.html)

The corrosion failure behavior of marine steel is affected by stress, which exists in offshore structures at sea-mud region. The sulfate reducing bacteria (SRB) in the sea-mud made the steel more sensitive to stress corrosion cracking (SCC) and weaken the corrosion fatigue endurance. In this paper, a kind of natural sea-mud containing SRB was collected. Both SCC tests by slow strain rate technique and corrosion fatigue tests were performed on a kind of selected steel in sea-mud with and without SRB at corrosion and cathodic potentials. After this, the electrochemical response of static and cyclic stress of the specimen with and without cracks in sea-mud was analyzed in order to explain the failure mechanism. Hydrogen permeation tests were also performed in the sea-mud at corrosion and cathodic potentials. It is concluded that the effect of SRB on environment sensitive fracture maybe explained as the consequences of the acceleration of SRB on corrosion rate and hydrogen entry into the metal. []

3.2.4 Carbon Dioxide

“The oceans play an important role in regulating the amount of CO 2 in the atmosphere because CO 2 can move quickly into and out of the oceans. Once in the oceans, the CO 2 no longer traps heat. CO 2 also moves quickly between the atmosphere and the land biosphere (material that is or was living on land).

Of the three places where carbon is stored—atmosphere, oceans, and land biosphere—approximately 93 percent of the CO 2 is found in the oceans. 

The oceans contain about 50 times more CO 2 than the atmosphere and 19 times more than the land biosphere. CO 2 moves between the atmosphere and the ocean by molecular diffusion when there is a difference between CO 2 gas pressure (pCO 2 ) between the atmosphere and oceans. For example, when the atmospheric pCO 2 is higher than the surface ocean, CO 2 diffuses across the air-sea boundary into the sea water.

The oceans are able to hold much more carbon than the atmosphere because most of the CO 2 that diffuses into the oceans reacts with the water to form carbonic acid and its dissociation products, bicarbonate and carbonate ions . The conversion of CO 2 gas into nongaseous forms such as carbonic acid and bicarbonate and carbonate ions effectively reduces the CO 2 gas pressure in the water, thereby allowing more diffusion from the atmosphere.

The oceans are mixed much more slowly than the atmosphere, so there are large horizontal and vertical changes in CO 2 concentration. In general, tropical waters release CO 2 to the atmosphere, whereas high-latitude oceans take up CO 2 from the atmosphere. CO 2 is also about 10 percent higher in the deep ocean than at the surface. The two basic mechanisms that control the distribution of carbon in the oceans are referred to as the solubility pump and the biological pump.”[]

3.2.5 Biological and Microbiological Environments

“Microbial corrosion, also called bacterial corrosion, bio-corrosion, microbiologically influenced corrosion, or microbially induced corrosion (MIC), is corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials.”[]

Microorganisms affect direct or indirect the integrity of materials used in offshore industry, frequently metals: iron, copper, nickel, aluminum, and their alloys. ,,Only titanium and its alloys appear to be generally resistant. “ []

Fig 3.11 MIC

Sursa:

Fig 3.12 Shallow stair-stepped MIC pit (left) vs. Smooth deep oxygen pit (right)

Sursa:

Chapter 4. Corrosion in the Black Sea

Seawater is a unique environment. Seawater can vary widely in terms of chemical composition, dissolved oxygen content, temperature, salinity, pH, carbonate levels, flow, degree of fouling, biological activity and pollution.

4.1 Temperature

The corrosion reaction rate in seawater increases as the temperature is increased. This applies only when the effect of temperature alone is a factor and other variables such as oxygen concentration, diffusion rates, salinity, calcareous deposit formation, and biological activity vary as a function of temperature and must also be considered as to how it affects the overall corrosion rate of a material, component, or system.

Metal Corrosion rates are usually higher at the surface where temperatures are warmer than in the deep ocean where the temperatures are colder.

Though increasing temperatures do tend to increase corrosion rates, rising temperatures also promote the formation of calcareous deposits and microbiological colonization, which from barrier layers that often throttle corrosion attack. Bacteria may participate in anaerboic corrosion reactions that replace the cathodic, oxygen reduction reaction.

Temperatures variations due to seasonal changes may also affect corrosion rates in seawater. For many copper and iron alloys, the corrosion rates increase duting the warmer months and year. Corrosion depends of the seawater environment.

For my project I used SeaDataNet, an efficient distributed Marine Data Management Infrastructure. The SeaDataNet infrastructure links already 90 national oceanographic data centres and marine data centres from 35 countries riparian to all European seas. The data centres manage large sets of marine and ocean data, originating from their own institutes and from other parties in their country, in a variety of data management systems and configurations. I selected a point on the map with coordonates: 30.2 longitude, 44.1latitude

4.2 Salinity

Salinity is defined as the total weight of solids dissolved in 1000g of water. Salinity usually is determinated by measuring chlorinity and then deriving the salinity:

As the salinity is increased, the chloride ion activity, the chloride ion activity increases and can result in increased pit and crevice corrosion initiation and propagation.

4.3 Dissolved Oxygen

Dissolved oxygen in seawater has a major influence on corrositivity since oxygen is the principal reactant involved in the cathodic reaction and is involved in the passivation reactions that occur for most metals and alloys in seawater.

The solubility of oxygen decreases as the temperature is increased.

Oxygen solubility in surface seawater can become supersaturated due to photosynthesis by marine plants and by wave action. The dissolved oxygen concentration can decrease and become undersaturated due to oxygen consumption created by the decomposition of organic matter.

Seawater temperature and salinity also affect the disolved oxygen content, with temperature having the larger influence on the oxygen solubility. If the temperature or the salinity is decreased, the dissolved oxygen content increases. The equilibrium concentration of dissolved oxygen in seawater as a function of salinity and temperature is shown in the next table:

The levels of the dissolved oxygen level on corrosion is dependent on the metal.

For metals that form passive films likestainless steel and aluminium, a high oxygen content is favorable in that it helps to delay the initiation of pitting on the metal surface.However, once pitting is initiated, the propagation rate is increased with high oxygen content in the seawater.

For common steels and copper alloys, the effect of dissolved oxygen is dependent on the seawater velocity. The oxygen concentration has a negligible effect on copper alloy corrosion rates in quiescent seawater while at a seawater flow rate of 1.8 m/sec, an increasing oxygen content accelerates the copper corrosion rate.

For metals such as iron and steel, the corrosion rate increases linearly with a rising oxygen concentration at a constant temperature.

Oxygen concentration cells can be created from deposits on a metal’s surface or many due to improper component design, which introduces crevices. Differences in oxygen content due to discrete films or deposits randomly scattered along a metal surface can cause pitting and/or crevice corrosion at these localized sites. In contrast, complete coverage of a surface by a film or a deposit can provide an effective barrier and reduce corrosion of the metal.

4.4 pH and Carbonates

Generally pH affect the corrosion of aluminum. Depending on various conditions, calcareous deposits form when the solubility of calcium and magnesium ions is exceeded.

Changes in pH do not directly alter the corrosion behavior of most metals, pHvariations can influence the formation of protective calcareous scales that subsequently can affect the corrosion rate of the substrate alloy.

Spontaneous calcareous deposits are more likely to occur at higher pH values, but the function of these deposits is limited due to the presence of organic matter and seawater salinity.

In early summer, the pH level on the surface of the Black Sea reaches 9.25, but for the mathematical model the value choosen is , the medium value.

4.5 Biological Organisms

Seawater is a living medium sustaining a wide variety of organisms. Marine biological organisms consist of either micro (bacterial) or macro (algae, barnacles) speciation.

These organisms can affect the corrosion behavior of metals and alloys in a number of ways including:

(a) influencing either or both the prevailing anodic and cathodic Reactions

(b) influencing the formation and/or maintenance

of protective films

(c) producing deposits on metal surfaces,

(d) creating a corrosive environment.

One of the keys to alternating conditions at the metallic surface, and the subsequent delay or acceleration of corrosion is the formation of a biofilm. The exact sequence of events leading to biofilm formation depends on the environment and the organisms present. Biological organisms attach and multiply on any solid surface in seawater. Within two hours of immersion, a non-living organic conditioning film develops on a solid surface.

Within the first 1-2 days, a bacterial slime film develops over the conditioning film. The slime film creates a partial barrier to diffusion between the liquid/metal interface and the bulk seawater environment. These slime films are usually not continuous and can create oxygen or chemical concentration cells on the metal surface, which can result in accelerated localized corrosion.

Biofilms are structured, but complex assemblages of microorganisms embedded in exopolymers (extracellular material produced by microorganisms that define their shape, their ability to adhere to solid surfaces, and their ability to trap particulate nutrients).

Biofilms include communities of colonies, consortia, newly created cells, dying cells, extracellular products, polymers, and trapped inorganic material. Biofilms and individual organisms that are contained within the biofilm interact with external influences originating from the substratum and the bulk medium to cause oxygen or chemical concentration gradients and other localized effects related to mitigating or accelerating corrosion.

Ammonia and sulfides can also be produced from the decay of organic matter within the slime film, resulting in increased corrosion of some alloys. Sulfur-oxidizing organisms produce sulfuric acid from sulfur or other reduced sulfur species. The presence of ammonia is known to cause stress corrosion cracking of copper alloys, while sulfides may lead to accelerated attack on copper alloys and steel. The presence of the slime film on the metal surface can locally change the local environment at the liquid/metal interface such that the corrosion behavior of a metal can be considerably altered from one that normally displays low corrosion rates in seawater to conditions where corrosion is accelerated.

The reduction or complete elimination of marine organisms in dilute seawater reduces the probability of forming protective biofilms and could result in increased metal corrosion.

The reduction or complete elimination of marine organisms in dilute seawater reduces the probability of forming protective biofilms and could result in increased metal corrosion.

Macrofouling films begin to develop over the conditioning and slime films within 2-3 days after immersion in seawater. If the macrofouling is continuous along the solid surface, the film can decrease the amount of dissolved oxygen present at the surface and thus decrease the overall metal corrosion rate. If, however, the macrofouling is discontinuous, the film acts in the same manner as discontinuous slime films where oxygen or chemical concentration cells can be formed and result in localized corrosion of the metal.

4.6 Velovity of Seawater

Fluid velocity can significantly affect metal corrosion rates in seawater. Flow-assisted corrosion is dependent on variables such as water chemistry, pH, component geometry, surface roughness, biofouling, microbiologically influenced corrosion, pitting and crevice corrosion, water pollution and contamination, alloy composition and surface films, galvanic interaction, fluid velocity and mode, oxygen content, heat transfer rate, and temperature .

Corrosion rates and the type of corrosion are often dependent on environmental factors such as fluid flow and the availability of appropriate species required to drive electrochemical reactions . A change in the motion of a corroding metal or alloy relative to its environment by fluid flow can increase corrosion rates by removing protective films or by increasing the diffusion or migration of deleterious species. However, an increase in fluid flow can decrease corrosion rates by eliminating aggressive ion concentration or enhancing passivation or inhibition by transporting the protective species to the fluid/metal interface.

Under high flow conditions, corrosion may take the form of impingement, erosion corrosion, or cavitation. In any given velocity domain, different local velocities may exist over diverse areas of the component due to factors such as geometry or mode of fabrication.

The effect of seawater velocity on a metal's corrosion rate varies with the particular alloy. Alloys such as 304 and 316 stainless steel or nickel-chromium-molybdenum alloys exhibit deep pitting in low flow conditions, yet at high seawater velocities their corrosion rate decreases to less than 25 pm per year. Contrary to this, iron and copper show significantly lower corrosion rates at low flow velocities than under high seawater flow conditions.

4.7 Sulfides

Sulfides, sulfur, and other sulfur compounds can produce pitting, crevice corrosion, dealloying, stress corrosion cracking, and stress-assisted hydrogen induced cracking of susceptible metals and alloys. Polluted waters, like those found in coastal harbors and estuaries, contain hydrogen

sulfide and sulfur-containing compounds. Sulfides can be generated in several ways:

(1) bacterial reduction of naturally occurring sulfates in seawater;

(2) rotting vegetation;

(3) industrial waste discharge .

Hydrogen sulfides are known to adversely affect the corrosion rate of various metals and alloys. Sulfates themselves are not usually harmful; however, sulfates can be reduced to harmful sulfides by sulfate-reducing bacteria. These sulfide pollutants can contribute to the corrosion of steels, stainless steels, copper, and aluminum alloys.

Sulfide corrosion has been found to occur on a number of different copper-base alloys. Wrought 90/10 copper-nickel can exhibit sulfide-induced attack in the form of accelerated pitting with as little as 0.01 ppm sulfide concentration while 70/30 copper-nickel is susceptible at sulfide concentrations of 0.05 ppm or greater.

The presence of sulfidemodified films interferes with the formation of normal passivating films of copper nickel alloys found in unpolluted waters. Sulfide-modified films generally are more loosely adherent than the normal cuprous oxide films; turbulence tends to selectively remove the sulfide-modified films. The accelerated corrosion rates of copper-nickel alloys

in aerated sulfide-containing seawater remain high since the sulfides prevent protective corrosion product layers from forming.

4.8 Heavy Metals

Concentrations of copper in seawater can cause accelerated corrosion of metallic components. In unpolluted seawater, the copper concentration is 0.2 ppb, which is typically not high enough to influence a metal's corrosion behavior. If, however, the copper concentration is increased above approximately 30 ppb, corrosion of aluminum alloys can occur. The copper concentration may be increased due to a number of factors including copper leaching from antifouling paints, pollutants containing copper, or nearby copper alloy corrosion. If the copper deposits onto aluminum.

The Black Sea is contaminated by trace metals. Heavy metals like cadmium, mercury, and lead, referred to as the priority pollutants in marine waters, do not present a threat to the marine ecosystem. The concentrations of heavy metals in bottom sediments and biota, collected in areas of influence of rivers and nearby ports and priority point pollution sources, are usually higher although decreasing or no trends have been observed. Accurate assessments of trends in biota and sediments are limited due to lack of long-term observations.

In Romanian coastal waters, levels of trace metals in the bottom sediment were monitored randomly and there are insufficient data to detect any trends.

http://www.blacksea-commission.org/_publ-SOE2002-eng.asp

Chapter 5. Examples of mathematical modelling of long term general corrosion of structural steels in the Black Sea

I create a diagram with FREECORP V1.0 that allow us to know the uniform corrosion of carbon steel at a single point in an environment containing carbon dioxide, acetic acid, oxygen, and/or hydrogen sulfide.

Conclusions

Annexes

References