Stainless Steel Properties and Corrosion
Stainless Steel Properties and Corrosion
The stainless steel are a family of ferrous alloys containing at least 11% chromium. The effect of this amount of chromium in steel in reducing corrosion is dramatic.The stainless steels exhibit two potential states:
The passive state is substantially cathodic to the active state. It is this potential difference that drives non-uniform attack, such as pitting and crevice corrosion on these alloys. For purposes of evaluating possible galvanic corrosion between the stainless steels grades and other alloys, the potential of the passive state should be used. As these alloys polarize readily, their potentials can have a wide range in some conditions and may be of little value in assessing galvanic corrosion problems based on field measurements.
Categories of alloys
There are many grades of stainless steels. By varying the chemical composition, heat treating, and cold-working, a wide range of properties is achieved. Wrought stainless steels are designated by a three digit number. The numbering system is applied by the American Iron and Steel Institute (AISI). The symbols designating cast stainless steels are defined by the alloy casting institute (ACI). Corrosion resistant grades have designations with the prefix C; heat resistant grades have designations with the prefix H.
The austenitic grades of wrought stainless steel are designated by numbers in the 200 and 300 series. The 300 series is based upon the classic 18% chromium and 8% nickel stainless steel. Certain modifications of the composition affect the corrosion resistance, whereas other modifications of the composition are made to achieve mechanical properties.
200 Series Stainless Steels.
This group of alloys is similar to the more common 300 Series alloys described below as they are non-magnetic and have an austenitic structure. The basic alloy contains 17% chromium, 4% nickel and 7% manganese. Although these alloys are somewhat more resistant to attack than the more common 300 series stainless steels, their overall performance is similar. Some proprietary grades related to the 200 series, have performance equal or superior to the best 300 series stainless steels. These alloys are, however, not immune to attack and are very susceptible to concentration cell corrosion and pitting corrosion attack. When corrosion starts they usually corrode rapidly and nonuniformly. In seawater immersion, the incubation time for these alloys is in the range of 1 to 3 months with some of the Nitronic grades having incubation times of up to 1 year.
In the 200 series of austenitic stainless steel, some of the nickel in the basic 18/8 composition is replaced with manganese and nitrogen.
300 Series Stainless Steels.
This group of alloys are non-magnetic and have an austenitic structure. The basic alloy contains 18% chromium and 8% nickel. These alloys are subject to crevice corrosion and pitting. They have a range of incubation times in seawater ranging from essentially zero in the case of the free machining grades, such as Type 303, to 6 months to 1 year for the best alloys, such as Type 316. They have been widely used in facilities with mixed results. If used in an application where chloride levels are low or where concentration cell corrosion has been prevented through design, they are likely to perform well. When chloride levels are high and where concentration cells can occur, the performance of these alloys is often poor. They must always be selected with care for a specific application and the effect of potential non-uniform attack, like pitting and crevice cororsion or stress corrosion cracking, on system performance must be addressed.
400 Series Stainless Steels.
This group of alloys are magnetic and have a martensitic structure. The basic alloy contains 11% chromium and 1% manganese. These alloys can be hardened by heat treatment but have poor resistance to corrosion. They are subject to both uniform and non-uniform attack in seawater. The incubation time for non-uniform corrosion attack in chloride containing environments is very short, often only hours or a few days. Unless protected, using these alloys in seawater or other environments where they are susceptible to corrosion is not recommended.
600 Series Stainless Steels.
This series of stainless steels is commonly referred to as “Precipitation Hardening” stainless steels. These steels can be heat treated to high strength levels. They are subject to crevice corrosion and pitting in chloride containing environments and are also subject to stress corrosion cracking and hydrogen embrittlement.
The incubation time for crevice corrosion and pitting corrosion of stainless steel in seawater is relatively short, often only a few days. The incubation time for stress corrosion cracking can be very short, sometimes measured in hours. The use of these alloys in chloride containing environments is not normally recommended unless they are carefully selected, their heat treatment is carefully specified and controlled, and the effect of pitting and crevice corrosion is properly addressed.
Miscellaneous Cast and Wrought Stainless Steels.
Alloy 20, a proprietary cast and wrought stainless steel has superior corrosion resistance to Type 316 stainless steel.
Many newly developed proprietary stainless steels have given good service in many applications and have corrosion resistance superior to Type 316 stainless steel. The use of these alloys must be evaluated based upon their specific corrosion properties. While the resistance of these alloys is superior to many other grades of stainless steels, the benefits of using these alloys in critical applications instead of more corrosion resistant or corrosion immune alloys must be balanced against the consequences of failure.
Duplex stainless steels contain both austenite and ferrite phases and about 22/25% chromium and 5/7% nickel.
Some of the 400 series stainless steels have a ferritic structure, and some alloys have a martensitic structure.
Martensitic stainless steels in the 400 series have chromium concentrations in the range of 11.5 to 18%, and they have an austenitic structure at high temperature that is transformed into marten site upon cooling to room temperature.
Stainless steels hardened by transformation to martensite are tempered to give the desired engineering properties. This tempering can influence corrosion susceptibility.
For example, corrosion susceptibility of type 420 stainless steel is at its maximum when the alloy is tempered at temperatures in the range of 450° to 600° C (842° to 1112°F).
Precipitation-hardening stainless steels are usually designated by a trade name rather than by their AISI 600 series designations.
There are three types of precipitation hardening stainless steels:
1. Martensitic types, which are supplied in the martensitic condition, are hardened by a simple aging treatment of the fabricated part.
2. Semi austenitic types, which are supplied in the austenitic condition, are transformed to martensite by special heat treatment before precipitation hardening.
3. The austenite in the austenitic types is precipitation hardened directly.
The heat treatments of precipitation-hardening stainless steels are chosen to optimize mechanical properties.
Precipitation hardening generally results in a slight increase in corrosion susceptibility and an increased susceptibility to hydrogen embrittlement.
source: MATERIAL DETERIORATION PREVENTION AND CONTROL GUIDE OF ARMY MATERIAL, PARTONE, METALS (MlL HDBK-73S)