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Stainless Steel

 
Pitting Corrosion | Crevice Corrosion | Stress Corrosion Cracking | Cleaning Stainless Steel | Magnetic Properties of Stainless Steel  |  Galvanic Corrosion | How to clean Stainless Steel | Who Discovered Mars | Who Discovered Iron | Sorting Stainless Steel | L & H grades of Stainless Steels | Galvanic Corrosion table | Steel Gauge Chart | Define Oxidation | Chemical Properties of Copper | Introduction to welding | What is the densities of Steel | Iron Corrosion | Stainless Steel Corrosion            
                                           
Stainless Steels.

S
tainless steels can be defined as a ferrous alloys that contain more than 11% chromium and are resistant to general rusting in mild atmospheric conditions. They are generally subject to non-uniform attack in chloride containing environments with crevice corrosion and pitting attack being common on many alloys. Due to the high rate of localized attack that occurs at the start of the attack, the incubation time for attack is often a more significant measure of corrosion resistance than penetration rates.

The stainless steels exhibit two potential states:

  • active
  • passive

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 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.


 

These videos from EuroInox explain the Corrosion Resistance Properties of Stainless Steel


The Self Passivation of Stainless Steel


Stainless Steel Self Repairing


 

 

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.

 

 

 

 

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 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 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.


 

Other info on Stainless Steels:


Design Guidelines for the selection and use of Stainless Steel

Stainless Steel corrosion tables

 

Stainless Steel corrosion by waters

 

Stainless Steel properties

 

Stainless Steel Welding