When it comes to corrosion engineering, 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. Here we'll look at the important factors that affect the corrosion of metals in seawater and the application of structural materials in seawater.
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:
- 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.
- 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.
- 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, as shown in the following figure:
The figure above shows how the combination of chloride concentration and dissolved oxygen concentration results in the maximum corrosion rate.
Since the pH of seawater varies in the range of 7.5 to 8.5, the oxygen reduction reaction is the predominant cathodic reaction in competition with the hydrogen evolution reaction. In fact, dissolved oxygen can have a significant influence on the corrosion rate of metals in seawater.
There are several factors that can influence the oxygen cathodic reaction. The agitation of seawater due to waves can increase the oxygen concentration in the seawater. Temperature is the other factor, and it produces two opposing effects. At first view, temperature can have an impact on the solubility of dissolved oxygen and the diffusion rate of dissolved oxygen. The diffusion rate of oxygen in seawater increases as the temperature of the water rises. In response, the corrosion rate increases as a result of the increasing limit current density of the oxygen reduction cathodic reaction. On the other hand, at high temperatures, the solubility of oxygen in seawater decreases. This effect can reduce the corrosion rate. Despite the above-mentioned effects, the solubility of oxygen in saline water is not affected by temperature as much as the diffusion coefficient is affected.
Moreover, salinity can influence dissolved oxygen concentration. Generally, the maximum oxygen concentration is obtained at 3.5 weight percent of NaCl.
Temperature is a factor that can impact both the activation polarization and concentration polarization, which can increase corrosivity in most types of corrosion. For example, the corrosion of steel in water will increase from 2 to 4% per 1.5°F (1°C) increase. Therefore, seawater corrosion in tropical areas is more significant than in arctic zones.
Structural Materials in Seawater
Here we'll take a look at the corrosion behavior of most important structural materials in seawater.
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.
Source: 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 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.
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.
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.
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.
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.