Seawater corrosion is caused by three factors: temperature, oxygen, and chloride concentration.


An Introduction to Pipeline Corrosion of Seawater


Seawater is a critical environment for corrosion engineering. This is especially true for pipelines that traverse the ocean carrying oil and natural gas around the world. They can cause the greatest problems if they fail. (Read more about this topic in Industry Experts Discuss subsea pipeline corrosion management. We'll be discussing the factors that influence the corrosion of metals and the application structural materials to saltwater.




Three Factors That Are Essential To Seawater Corrosion


1. Chloride Concentration


Saltwater contains the most aggressive substance in seawater, chloride ions. The concentration of chloride in water is often called "salinity". This is usually found in seawater. It varies depending on solar evaporation, precipitation, and the dilution by freshwater and circulation.



Three factors can explain the corrosivity in seawater of chloride ions:




The following reactions can be performed by chloride ions and dissolved ferrous ions to make ferrous chloride:




Fe = Fe2+ + 2 E-


Fe2+ + 2 CL- = FeCl2




Ferrous chloride can be formed in this reaction by reacting with dissolved oxygen to produce ferric oxide and ferric chloride (Fe2O3). This is an oxidizing agent which can increase the general corrosion rate as well as pitting corrosion. Ferric ions can cause severe corrosion by shifting the corrosion potential (Ecorr), to higher values than Eb (pitting or breakdown potential).


Chloride ions, also known as "aggressive ions", can affect pit growth and pit initiation. They can penetrate the passive layer and increase pit initiation risks. Chlorides can also worsen pit growth by an autocatalytic process. (Refer to Related Reading: How to Effectively Recognize and Prevent Pitting Corrosion.




Pitting corrosion can only occur if there is stagnant water. Pitting corrosion will not occur in areas where water is constantly being replaced and moving.


Another important factor that influences the corrosivity is dissolved oxygen. The solubility or insolubleness of oxygen in seawater can be affected by the concentration of chlorides. As shown in the following graph, a maximum oxygen concentration of 3.5 percent sodium chloride is possible.


Figure 1. Figure 1.




2. Oxygen


The pH ranges from 7.5 to 8.5, so the oxygen reduction reaction is dominant in competition with hydrogen evolution. Actually, dissolved oxygen can play a significant role in the corrosion rate for metals in seawater.



The oxygen cathodic response can be affected by many factors. Waves can cause seawater to agitate, which can lead to an increase in oxygen concentration. The other factor is temperature, which can have two opposite effects. Temperature can impact the solubility and diffusion rates of dissolved oxygen. As seawater temperatures rise, the diffusion rate of oxygen in seawater is affected by the temperature. The oxygen reduction cathodic reaction increases the limit current density, which causes corrosion to increase. At high temperatures, however, the solubility in seawater of oxygen decreases. This can decrease the rate of corrosion. Despite these effects, the solubility and diffusion coefficient of oxygen in saline waters are not affected by temperature.




The salinity can also influence the concentration of dissolved oxygen. The maximum oxygen concentration is generally found at 3.5 percent NaCl.



3. Temperature


Temperature can have an impact on both activation polarization as well as concentration polarization. This can lead to increased corrosivity for most types of corrosion. The corrosion of steel in water can increase by 2 to 4 percent per 1.5degF (1degC). Seawater corrosion is much more severe in tropical regions than in the arctic.



Seawater Structural Materials


We'll be looking at corrosion behavior of some of the most important structural elements in seawater.



Plain Steels


Mild steels, if left unprotected in marine environments, are susceptible to corrosion. They are used in marine environments as sheet piles and ship bodies. After applying a suitable protection technique such as cathodic or polymeric coatings, they can be used. For a detailed case study, see How a Liquid Nynylon Coating can help create an impervious, pinhole-free coating barrier. The corrosion rate of an unprotected steel structure in saltwater is dependent on its location relative to the ocean. See the image below. The location of the steel structure relative to the ocean will affect the rate of corrosion.




The bottom of the sea (immersion area) is where the seawater stagnates and has the lowest oxygen and temperature. This area is likely to have a very low corrosion rate in comparison to other areas.


Higher levels are found at the bottom of sea level, which is known as the tidal area. Here materials are exposed to a cyclic drying-wetting process. This cycle occurs every 24 hours, which can increase the rate of corrosion. The corrosion rate for mild steel in this region is approximately 100 um/yr. However, this value is less than 50 um/yr and even close to zero according to previous research. The "splash area" is the level where the most severe corrosion occurs. This zone is caused by high temperatures, saturation with oxygen, and splashing of seawater. The splash zone corrosion rate can reach 900 um/yr (for instance, in Alaska's Cook Inlet).


The corrosion rate in the splash zone is slightly higher than that of other areas of the tidal area. The oxygen concentration cell is responsible for the higher corrosion rate. This cell has the anode located below the splash zone, where O2 partial pressure drops low, and the cathode at the splash area where O2 partial pressure rises high.




A thin layer of seawater can condense on the metallic surface at levels that are higher than the seasurface, which is known as the "marine environment". This can lead to atmospheric corrosion. Temperature, saltiness of seawater and wind intensity are all important factors that can affect marine atmosphere corrosion.




Painting, sheathing and cathodic protection are all good options to prevent corrosion of steel piles and columns in seawater.



Stainless Steels


Due to the protective chromium oxide coating, stainless steels are highly resistant to corrosion in seawater. These alloys can be pitted if they are exposed to high levels of chloride in seawater. For example, the common stainless steel stainless steel type 304 is not immune to seawater pitting corrosion. When stainless steel is chemically enriched with 2% molybdenum, the pitting resistance increases to an acceptable level. This results in stainlesssteel type 316. Similar to molybdenum and stainless steels with higher chromium levels can increase their resistance to pitting in still water.




Copper Alloys


The seawater corrosion resistance of copper and its alloys (bronze, brass, etc.) is usually high. For marine use, it is recommended to use diffident copper alloys.




Sometimes, brass alloys are modified in order to perform better in marine environments. For example, naval brass alloys or admiralty brass contain 1% tin to prevent zincification. Arsenical brass contains very low levels of arsenic to stop dezincification. To improve the corrosion resistance of brass alloys used in ship impellers, aluminum is often added to brass. Cupronickels, which are copper with 10-30% of nickel alloys, are popular for marine applications because they resist seawater corrosion. This article explains more about cupronickels and why you should be using it now.




Concrete


Concrete can be contaminated by chlorides through cracks and pore spaces. They may also touch reinforced steel rods that are passivated in concrete's alkaline environment. Localized corrosion can result. Concrete eventually corrodes due to the pressure from rust growth.




Aluminium


The alloying elements used and the surface finish are crucial in determining aluminum's corrosion resistance in marine environments. The aluminum's corrosion resistance is affected by the presence of copper and iron in the alloy. However, aluminum alloys of the 5xxx series, which contain magnesium, are often suitable for marine applications (such 5052 alloy). Marine corrosion can be prevented by creating a hard anodized (thick aluminum oxide) layer on the aluminum surface.




Titanium Alloys and Titanium


One of the most popular options for marine services is titanium and titanium alloys. Titanium, despite its high cost, should be considered.