Analysis of corrosion resistance of stainless steel and steel pipes

What is corrosion?
Corrosion is an electrochemical process in which a metal reacts with its surroundings, and the resulting degradation triggers a loss of material properties such as mechanical strength, appearance, and impermeability to liquids and gases. The cause of corrosion is the energy difference between the metal and its natural ore. It takes energy to extract any metal from the ore. This "excess energy" drives corrosion because the metal will try to return to its natural state. Electrochemically speaking, corrosion is the release of electrons. The process of releasing electrons is called an oxidation reaction or an anodic reaction. However, these electrons need to be consumed somewhere, so a reduction reaction or a cathodic reaction needs to occur.

Metals that are easily oxidized, such as magnesium, are called non-precious (active) metals, while the most corrosion-resistant metals, such as gold and platinum, are called precious (inert) metals. In between, we find most metals for engineering purposes, such as iron and copper.

Stainless steel and passivation
Stainless steel is not a precious metal like gold or platinum, by its nature. The corrosion resistance of stainless steel comes from an invisible and insoluble thin layer of chromium and iron oxides and hydroxides, commonly called a passive film.
Even if the passive film is only a few nanometers thick, it can effectively isolate the metal below from the surrounding environment, effectively slowing down the electrochemical reaction that causes corrosion, and the corrosion rate is much lower than that without a passive film. Other metals (such as chromium, aluminum, and titanium) also exhibit passivation, and stainless steel takes advantage of the ability of chromium to passivate.
In an environment containing sufficient oxidants, the passive film on the surface of stainless steel will form spontaneously. In addition, if the metal under the passive film is exposed due to mechanical damage (such as scratches), it will spontaneously re-passivate. The oxygen content in air and most aqueous solutions is sufficient to form and maintain the stainless steel passive film. If the passive film can be effectively retained, stainless steel can be used almost permanently.

Uniform corrosion
Uniform corrosion occurs in an environment where the passive film is unstable, and the unprotected metal surface is reduced more or less uniformly. Uniform corrosion of stainless steel is most common in acid or hot alkaline solutions. In addition, molten salts of chlorides and fluorides can also cause uniform corrosion.
In an environment with constant temperature and chemical composition, uniform corrosion is expected to occur at a reasonably constant rate. The corrosion rate can be determined by measuring the weight loss per unit of time over a certain surface area. This is usually expressed as a loss of thickness over time, such as mm/year. By definition, stainless steel is generally considered to be resistant to uniform corrosion in a particular environment if the corrosion rate does not exceed 0.1 mm/year.

Pitting and crevice corrosion
Uniform corrosion causes widespread destruction of the passive film, but pitting and crevice corrosion are caused by localized destruction of the passive film. In actual situations, corrosion failure of stainless steel is usually a result of localized corrosion rather than uniform corrosion. In this case, galvanic couples are formed locally on the surface of the stainless steel, which causes rapid corrosion propagation. Compared with uniform corrosion, the weight loss of localized corrosion may be small, and the corrosion rate as an indicator of corrosion severity is not related to localized corrosion. In contrast, pitting and crevice corrosion is considered to be an either-or situation, and once localized corrosion begins, it quickly penetrates the material and needs to be avoided.
In environments containing halogen ions, such as chlorides, stainless steel is particularly susceptible to pitting and crevice corrosion. Therefore, environments that present a risk of localized corrosion include liquids containing large amounts of chlorides, such as seawater and various industrial solutions.
Crevice corrosion occurs in crevices and other confined spaces, as well as under deposits formed during use. In aqueous environments, naturally occurring chemical reactions on the surface of stainless steel consume oxygen. In the stagnant solution inside the crevice, the supply of new oxidants is limited. The composition of the solution inside the crevice gradually becomes different from that of the surrounding solution. This composition difference increases the risk of corrosion when a concentration cell is formed.

The increasingly corrosive environment eventually breaks down the passive film inside the crevice, and the small area of exposed metal surface will act as an anode for the larger passive area around the crevice.
As with any type of corrosion, the risk of pitting and crevice corrosion depends on environmental factors and the corrosion resistance of the alloy. High chloride concentrations, low pH values, and high temperatures all increase the likelihood of pitting and crevice corrosion. Other halides, such as bromides and iodides, may also cause pitting and crevice corrosion.

Stress Corrosion Cracking
Stress corrosion cracking (SCC) is a brittle failure mode caused by the combined effects of mechanical stress and a corrosive environment. In environments where pitting, crevice, or uniform corrosion is not expected, it can cause a rapid loss of mechanical strength and may lead to catastrophic failure by cracking, making it a hidden form of corrosion. For stress corrosion cracking to occur, three requirements must be met:
• Susceptible material
• Environment in which the material is susceptible to stress corrosion cracking
• Sufficient tensile stress
If one of these three factors is removed, stress corrosion cracking will not occur. As with pitting and crevice corrosion, the most common causes of stress corrosion in stainless steel are chloride-containing solutions and elevated temperatures. The risk of stress corrosion cracking increases with increasing chloride concentrations, increasing temperatures, and decreasing pH. Stress corrosion in stainless steel usually appears in the form of small, branching cracks.

Failures caused by stress corrosion cracking often occur suddenly and without warning due to the rapid propagation of cracks. In the most severe cases, failure of a component can occur within days or even hours.

Corrosion fatigue
When a material is subjected to cyclic loading, it is capable of failing at loads far below the material's ultimate tensile stress. If the material is simultaneously exposed to a corrosive environment, failure may occur after a shorter time, even at lower loads. This is caused by a type of corrosion called corrosion fatigue, which is similar to stress corrosion cracking and leads to brittle failure. However, cracks caused by corrosion fatigue have fewer branching. Corrosion fatigue usually occurs in room temperature environments and neutral solutions.

Intergranular corrosion
Intergranular corrosion refers to a type of local corrosion in which the grain boundaries and adjacent parts of the material are preferentially corroded, while the grains themselves are not corroded or corroded very lightly, see Figure 7. In stainless steel, the precipitation of chromium carbides or intermetallic phases may cause intergranular corrosion. Due to the high carbon content of stainless steel in the past (0.05-0.15%), this corrosion is potentially dangerous. The use of AOD (argon oxygen refining furnace) in the production process of modern stainless steel has reduced the carbon content, so intergranular corrosion is rarely a problem today.

Galvanic corrosion
Galvanic corrosion is the electrochemical corrosion caused by two different metals in contact with each other while in an electrolyte. Usually, the more active metal (anode) will be more severely corroded, while the inert metal (cathode) is protected. This galvanic corrosion is usually most likely to occur near the junction of the two metals.
As long as stainless steels remain in their passive state, they will be more noble than other metallic materials in most environments and will therefore be the cathode in most galvanic couples. On the other hand, resistive coupling with stainless steel may increase the corrosion rate of non-precious metals such as high-carbon steel, galvanized steel, copper, and brass. Galvanic corrosion between different stainless steel grades is not usually a problem, provided that each stainless steel is passivated in the specific environment.