Corrosion resistant ability of stainless steel
Stainless steels have high corrosion resistant ability. In different grads, different surface treatments, and different environments the corrosion resistant ability changes. Here we list up three factors as examples to see how they affect the corrosion resistant ability of stainless steel.
- Elements
PREN is the Pitting Resistance Equivalent; this number is a measure of the relative pitting corrosion resistance of a stainless steel grade in a chloride containing environment. The higher the PREN the more corrosion resistant a stainless steel is. The formula for PREN is:
PREN = %Cr + 3.3*%Mo + 16*%N
This formula expresses that molybdenum is 3.3 times more effective against pitting corrosion than chromium. However, chromium is always necessary to give a base corrosion resistance. Molybdenum cannot replace a base amount of chromium in stainless steels. It boosts the corrosion resistance of a stainless steel.
The figure shows that ferritic, austenitic and duplex stainless steels are available on different levels of corrosion resistance. For example, Type 444 ferritic, Type 316 austenitic and 2304 duplex stainless steels all have a similar level of pitting corrosion resistance in chloride environments.(*0)
- Surface finish
- Environment
A major problem in the process industry is the corrosion of metals in pipes, valves and other parts of the constructions. This guide indicates acceptable combinations of more or less aggressive fluids and commonly used materials.
Note! Remember that corrosion is a complicated issue, depending on the combinations of materials and the fluids, the fluid temperatures, the surrounding environment and the galvanic currents in the constructions. The table below must be used with care. Always check with the producer of the material.(*3)
Surface finish does not influence bacterial attachment, colonisation, or removal, but is an important parameter for the corrosion resistance of the surface.(*1)
Chemical treatments such as pickling, passivation,and electropulishing have a benefit of increasing the corrosion resistance by removing impurities from the surface or imparting brilliance to its surface by removal of a thin surface layer.
Corrosion resistance may be reduced depending on the stainless steel grade used. By using grade 316 with a No 4 finish in aggressive environments, the corrosion resistance is negated and may be less than on 304 with a 2B or BA finish.
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| Figure 1: The acceleration of the corrosion of the surface at Ra above 0.5 micrometres is apparent. |
Figure 1 shows the results of electrochemical tests for corrosion of a polished surface. Corrosion resistance of a smooth surface can be better than the corrosion resistance of an abraded surface of a more highly alloyed grade. (*2)
| Corrosion Resistance 1)Good 2) Be Careful 3) Not Useable | ||||||||||||
| Fluid |
Metal | |||||||||||
| Carbon Steel | Cast Iron | 302 and 304 Stainless Steel | 316 Stainless Steel | Bronze | Durimet | Monel | Hasteloy B | Hasteloy C | Titanium | Cobalt base alloy 6 | 416 Stainless Steel | |
| Acetaldehyde | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||
| Acetic acid, air free | 3 | 3 | 2 | 2 | 2 | 1 | 2 | 1 | 1 | 1 | 1 | 3 |
| Acetic acid, aerated | 3 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 3 |
| Acetic acid, vapors | 3 | 3 | 1 | 1 | 2 | 2 | 2 | 1 | 1 | 1 | 3 | |
| Acetone | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Acetylene | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||
| Alcohols | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Aluminum Sulfate | 3 | 3 | 1 | 1 | 2 | 1 | 2 | 1 | 1 | 1 | 3 | |
| Ammonia | 1 | 1 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 1 | 1 | 1 |
| Ammonium chloride | 3 | 3 | 2 | 2 | 2 | 1 | 2 | 1 | 1 | 1 | 2 | 3 |
| Ammonium Nitrate | 1 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 1 | 1 | 3 |
| Ammonium Phosphate | 4 | 3 | 1 | 1 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 2 |
| Ammonium Sulfate | 3 | 3 | 2 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 3 |
| Ammonium Sulfite | 3 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 1 | 2 | |
| Aniline | 3 | 3 | 1 | 1 | 3 | 1 | 2 | 1 | 1 | 1 | 1 | 3 |
| Asphalt | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Beer | 2 | 2 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Benzene (benzol) | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Benzoic acid | 3 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||
| Boric acid | 3 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Butane | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Calcium Chloride (alkaline) | 2 | 2 | 3 | 2 | 3 | 1 | 1 | 1 | 1 | 1 | 3 | |
| Calcium hypochlorite | 3 | 3 | 2 | 2 | 2 | 1 | 2 | 3 | 1 | 1 | 3 | |
| Carbolic acid | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Carbon dioxide, dry | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Carbon dioxide, wet | 3 | 3 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Carbon disulfide | 1 | 1 | 1 | 1 | 3 | 1 | 2 | 1 | 1 | 1 | 1 | 2 |
| Carbon tetrachloride | 2 | 2 | 2 | 2 | 1 | 1 | 1 | 2 | 1 | 1 | 3 | |
| Carbonic acid | 3 | 3 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | ||
| Chlorine gas | 1 | 1 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 3 | 2 | 3 |
| Chlorine gas, wet | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 2 | 1 | 2 | 3 |
| Chlorine, liquid | 3 | 3 | 3 | 3 | 2 | 2 | 3 | 3 | 1 | 3 | 2 | 3 |
| Chromic acid | 3 | 3 | 3 | 2 | 3 | 3 | 1 | 3 | 1 | 1 | 2 | 3 |
| Citric acid | 3 | 2 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 2 | ||
| Coke oven gas | 1 | 1 | 1 | 1 | 2 | 1 | 2 | 1 | 1 | 1 | 1 | 1 |
| Copper sulfate | 3 | 3 | 2 | 2 | 2 | 1 | 3 | 1 | 1 | 1 | ||
| Cottonseed oil | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Creosote | 1 | 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Ethane | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Ether | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Ethyl chloride | 3 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Ethylene | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Ethylene glycol | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |||
| Ferric chloride | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 2 | 1 | 2 | 3 |
| Formaldehyde | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Formic acid | 3 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 3 | 2 | 3 | |
| Freon wet | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Freon dry | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Furfural | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Gasoline | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Glucose | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Hydrochloric acid, aerated | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 1 | 2 | 3 | 2 | 3 |
| Hydrochloric acid, air free | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 1 | 2 | 3 | 2 | 3 |
| Hydrofluoric acid, aerated | 2 | 3 | 3 | 2 | 3 | 2 | 3 | 1 | 1 | 3 | 2 | 3 |
| Hydrofluoric acid, air free | 1 | 3 | 3 | 2 | 3 | 2 | 1 | 1 | 1 | 3 | 3 | |
| Hydrogen | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Hydrogen peroxide | 1 | 1 | 1 | 3 | 1 | 1 | 2 | 2 | 1 | 2 | ||
| Hydrogen sulfide, liquid | 3 | 3 | 1 | 1 | 3 | 2 | 3 | 1 | 1 | 1 | 1 | 3 |
| Magnesium Hydroxide | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Mercury | 1 | 1 | 1 | 1 | 3 | 1 | 2 | 1 | 1 | 1 | 1 | 1 |
| Methanol | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Methyl ethyl ketone | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Milk | 3 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 3 |
| Natural gas | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Nitric acid | 3 | 3 | 1 | 2 | 3 | 1 | 3 | 3 | 2 | 1 | 3 | 3 |
| Oleic acid | 3 | 3 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Oxalic acid | 3 | 3 | 2 | 2 | 2 | 1 | 2 | 1 | 1 | 2 | 2 | 2 |
| Oxygen | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Petroleum oils | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Phosphoric acid, aerated | 3 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 2 | 1 | 3 |
| Phosphoric acid, air free | 3 | 3 | 1 | 1 | 3 | 1 | 2 | 1 | 1 | 2 | 1 | 3 |
| Phosphoric acid vapors | 3 | 3 | 2 | 2 | 3 | 1 | 3 | 1 | 2 | 3 | 3 | |
| Picric acid | 3 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 2 | ||
| Potassium chloride | 2 | 2 | 1 | 1 | 2 | 1 | 2 | 1 | 1 | 1 | 3 | |
| Potassium hydroxide | 2 | 2 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 2 | |
| Propane | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Rosin | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Silver Nitrate | 3 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 1 | 2 | 2 |
| Sodium acetate | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Sodium carbonate | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Sodium chloride | 3 | 3 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Sodium chromate | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Sodium hydroxide | 1 | 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Sodium hypochloride | 3 | 3 | 3 | 3 | 3 | 2 | 3 | 3 | 1 | 1 | 3 | |
| Sodium thiosulfate | 3 | 3 | 1 | 1 | 3 | 1 | 3 | 1 | 1 | 1 | 2 | |
| Stannous chloride | 2 | 2 | 3 | 1 | 3 | 1 | 2 | 1 | 1 | 1 | 3 | |
| Stearic acid | 1 | 3 | 1 | 1 | 2 | 1 | 2 | 1 | 1 | 1 | 2 | 2 |
| Sulfate liquor | 1 | 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | |
| Sulfur | 1 | 1 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Sulfur dioxide, dry | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 2 |
| Sulfur trioxide, dry | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 2 |
| Sulfuric acid, aerated | 3 | 3 | 3 | 3 | 3 | 1 | 3 | 1 | 1 | 2 | 2 | 3 |
| Sulfuric acid, air free | 3 | 3 | 3 | 3 | 2 | 1 | 2 | 1 | 1 | 2 | 2 | 3 |
| Sulfurous acid | 3 | 3 | 2 | 2 | 2 | 1 | 3 | 1 | 1 | 1 | 2 | 3 |
| Tar | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Trichloroethylene | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Turpentine | 2 | 2 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 |
| Vinegar | 3 | 3 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 3 | |
| Water, steam boiler feeding system | 2 | 3 | 1 | 1 | 3 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Water, distilled | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
| Water, sea | 2 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 3 |
| Whiskey | 3 | 3 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 3 |
| Wine | 3 | 3 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 3 |
| Zinc chloride | 3 | 3 | 3 | 3 | 3 | 1 | 3 | 1 | 1 | 1 | 2 | 3 |
| Zinc sulfate | 3 | 3 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 2 |
Remark stars:
*1 International Biodeterioration & Biodegradation 2003, Vol: 52 p. 175-185
*2 Australian Stainless magazine - Issue 36, Winter 2006.