Corrosion resistance

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Unlike carbon steel, stainless steels do not suffer uniform corrosion when exposed to wet environments. Unprotected carbon steel rusts readily when exposed to a combination of air and moisture. The resulting iron oxide surface layer is porous and fragile. In addition, as iron oxide occupies a larger volume than the original steel, this layer expands and tends to flake and fall away, exposing the underlying steel to further attack. In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in the air and even the small amount of dissolved oxygen in the water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing, even when scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade.

The resistance of this film to corrosion depends upon the chemical composition of the stainless steel, chiefly the chromium content. It is customary to distinguish between four forms of corrosion: uniform, localized (pitting), galvanic, and SCC (stress corrosion cracking). Any of these forms of corrosion can occur when the grade of stainless steel is not suited for the working environment.

The designation "CRES" refers to corrosion-resistant steel. Most, but not all, mentions of CRES refer to stainless steel—non-stainless steel materials can also be corrosion-resistant.

Uniform corrosionedit

Uniform corrosion takes place in very aggressive environments, typically where chemicals are produced or heavily used, such as in the pulp and paper industries. The entire surface of the steel is attacked, and the corrosion is expressed as corrosion rate in mm/year (usually less than 0.1 mm/year is acceptable for such cases). Corrosion tables provide guidelines.

This is typically the case when stainless steels are exposed to acidic or basic solutions. Whether stainless steel corrodes depends on the kind and concentration of acid or base and the solution temperature. Uniform corrosion is typically easy to avoid because of extensive published corrosion data or easily-performed laboratory corrosion testing.

Acidsedit

Acidic solutions can be put into two general categories: reducing acids, such as hydrochloric acid and dilute sulfuric acid, and oxidizing acids, such as nitric acid and concentrated sulfuric acid. Increasing chromium and molybdenum content provides increased resistance to reducing acids while increasing chromium and silicon content provides increased resistance to oxidizing acids.

Sulfuric acid is one of the most-produced industrial chemicals. At room temperature, Type 304 stainless steel is only resistant to 3% acid, while Type 316 is resistant to 3% acid up to 50 °C (122 °F) and 20% acid at room temperature. Thus Type 304 SS is rarely used in contact with sulfuric acid. Type 904L and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature. Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid, and thus silicon-bearing stainless steels are also useful.citation needed

Hydrochloric acid damages any kind of stainless steel and should be avoided.:118

All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentrations and elevated temperatures, attack will occur, and higher-alloy stainless steels are required.

In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid. As the molecular weight of organic acids increases, their corrosivity decreases. Formic acid has the lowest molecular weight and is a weak acid. Type 304 can be used with formic acid, though it tends to discolor the solution. Type 316 is commonly used for storing and handling acetic acid, a commercially important organic acid.

Basesedit

Type 304 and Type 316 stainless steels are unaffected weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking. Increasing chromium and nickel contents provide increased resistance.

Organicsedit

All grades resist damage from aldehydes and amines, though in the latter case Type 316 is preferable to Type 304; cellulose acetate damages Type 304 unless the temperature is kept low. Fats and fatty acids only affect Type 304 at temperatures above 150 °C (302 °F) and Type 316 SS above 260 °C (500 °F), while Type 317 SS is unaffected at all temperatures. Type 316L is required for the processing of urea.page needed

Localized corrosionedit

Localized corrosion can occur in several ways, e.g. pitting corrosion and crevice corrosion. These localized attacks are most common in the presence of chloride ions. Higher chloride levels require more highly-alloyed stainless steels.

Localized corrosion can be difficult to predict because it is dependent on many factors, including:

• Chloride ion concentration. Even when chloride solution concentration is known, it is still possible for localized corrosion to occur unexpectedly. Chloride ions can become unevenly concentrated in certain areas, such as in crevices (e.g. under gaskets) or on surfaces in vapor spaces due to evaporation and condensation.
• Temperature: increasing temperature increases susceptibility.
• Acidity: increasing acidity increases susceptibility.
• Stagnation: stagnant conditions increase susceptibility.
• Oxidizing species: the presence of oxidizing species, such as ferric and cupric ions, increases susceptibility.

Pitting corrosion resistanceedit

Pitting corrosion is considered the most common form of localized corrosion. The corrosion resistance of stainless steels to pitting corrosion is often expressed by the PREN, obtained through the formula:

${\displaystyle \text{PREN}=\mathrm{\%<!--\; \%\; -->}\text{Cr}+3.3\cdot <!--\; \cdot \; -->\mathrm{\%<!--\; \%\; -->}\text{Mo}+16\cdot <!--\; \cdot \; -->\mathrm{\%<!--\; \%\; -->}\text{N}}$,

where the terms correspond to the proportion of the contents by mass of chromium, molybdenum, and nitrogen in the steel. For example, if the steel consisted of 15% chromium %Cr would be equal to 15.

The higher the PREN, the higher the pitting corrosion resistance. Thus, increasing chromium, molybdenum, and nitrogen contents provide better resistance to pitting corrosion.

Crevice corrosionedit

Though the PREN of a certain steel may be theoretically sufficient to resist pitting corrosion, crevice corrosion can still occur when poor design has created confined areas (overlapping plates, washer-plate interfaces, etc.) or when deposits form on the material. In these select areas, the PREN may not high enough for the service conditions. Good design and fabrication techniques combined with correct alloy selection can prevent such corrosion.

Stress corrosion crackingedit

Stress corrosion cracking (SCC) is a sudden cracking and failure of a component without deformation.

It may occur when three conditions are met:

• The part is stressed (by an applied load or by residual stress).
• The environment is aggressive (high chloride level, temperature above 50 °C (122 °F), presence of H₂S).
• The stainless steel is not sufficiently SCC-resistant.

The SCC mechanism results from the following sequence of events:

1. Pitting occurs.
2. Cracks start from a pit initiation site.
3. Cracks then propagate through the metal in a transgranular or intergranular mode.
4. Failure occurs.

Whereas pitting usually leads to unsightly surfaces and, at worst, to perforation of the stainless sheet, failure by SCC can have severe consequences. It is therefore considered as a special form of corrosion.

As SCC requires several conditions to be met, it can be counteracted with relatively easy measures, including:

• Reducing the stress level (the oil and gas specifications provide requirements for maximal stress level in H₂S-containing environments).
• Assessing the aggressiveness of the environment (high chloride content, temperature above 50 °C (122 °F), etc.).
• Selecting the right type of stainless steel: super austenitic such as grade 904L or super-duplex (ferritic stainless steels and duplex stainless steels are very resistant to SCC).

Galvanic corrosionedit

Galvanic corrosion (also called "dissimilar-metal corrosion") refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. The most common electrolyte is water, ranging from freshwater to seawater. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would alone, while the other becomes the cathode and corrodes slower than it would alone. Stainless steel, due to having a more positive electrode potential than for example carbon steel and aluminium, becomes the cathode, accelerating the corrosion of the anodic metal. An example is the corrosion of aluminium rivets fastening stainless steel sheets in contact with water.

The relative surface areas of the anode and the cathode are important in determining the rate of corrosion. In the above example, the surface area of the rivets is small compared to that of the stainless steel sheet, resulting in rapid corrosion. However, if stainless steel fasteners are used to assemble aluminium sheets, galvanic corrosion will be much slower because the galvanic current density on the aluminium surface will be an order of magnitude smaller. A frequent mistake is to assemble stainless steel plates with carbon steel fasteners; whereas using stainless steel to fasten carbon-steel plates is usually acceptable, the reverse is not.

Providing electrical insulation between the dissimilar metals, where possible, is effective at preventing this type of corrosion.

High-temperature corrosion (scaling)edit

At elevated temperatures, all metals react with hot gases. The most common high-temperature gaseous mixture is air, of which oxygen is the most reactive component. To avoid corrosion in air, carbon steel is limited to approximately 480 °C (900 °F). Oxidation resistance in stainless steels increases with additions of chromium, silicon, and aluminium. Small additions of cerium and yttrium increase the adhesion of the oxide layer on the surface.

The addition of chromium remains the most common method to increase high-temperature corrosion resistance in stainless steels; chromium reacts with oxygen to form a chromium oxide scale, which reduces oxygen diffusion into the material. The minimum 10.5% chromium in stainless steels provides resistance to approximately 700 °C (1,300 °F), while 16% chromium provides resistance up to approximately 1,200 °C (2,200 °F). Type 304, the most common grade of stainless steel with 18% chromium, is resistant to approximately 870 °C (1,600 °F). Other gases, such as sulfur dioxide, hydrogen sulfide, carbon monoxide, chlorine, also attack stainless steel. Resistance to other gases is dependent on the type of gas, the temperature, and the alloying content of the stainless steel.

With the addition of up to 5% aluminium, ferritic grades Fr-Cr-Al are designed for electrical resistance and oxidation resistance at elevated temperatures. Such alloys include Kanthal, produced in the form of wire or ribbons.