The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a “passive” film which resists further “oxidation” or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become “passivated” and the oxidation or “rusting” rate will slow down to less than 0.002″ per year (0,05 mm. per year).
Unlike aluminum or silver this passive film is invisible in stainless steel. It’s created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called “ceramic”. This protective oxide or ceramic coating is common to most corrosion resistant materials.
Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters “ine”. Listed in order of their activity they are:
- fluorine
- chlorine
- bromine
- iodine
- astatine (very unstable.)
Chlorides are one of the most common elements in nature and if that isn’t bad enough, they’re also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack.
The most common forms of corrosion in stainless steel are:
| TYPE OF CORROSION | General Cause | Assessment/Treatment |
|---|---|---|
| Pitting | This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. This difference in relative areas accelerates the corrosion, causing the pits to penetrate deeper. halogens will penetrate passivated stainless steel. Listed in order of activity they are:
Passive layer attacked e.g. by chloride ion in salt or bleach | Calculate Pitting Resistance Equivalent Number from Alloy content |
| Crevice | Oxygen starvation to passive layer due to tight crevices This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash;9.0) and its high chloride content. | Use corrosion resistant grade of steel, or use flexible sealant |
| General | General breakdown of passive layer caused by chemical attack, often Hydrochloric or Sulphuric acid | The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part. Control concentration and temperature |
| Stress Corrosion Cracking (SCC) | Rare combination of tensile stress, temperature and corrosive effect such as Chloride Ions | Stress cracking can be minimized by annealing the metal, after manufacture, to remove residual manufactured stresses. Never replace a carbon steel bolt with a stainless steel one unless you’re sure there are no chlorides present. Bolts can be under severe tensile stress. No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature. Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane, explaining the reason we sometimes experience cracks after we weld stainless steel and dye check it to inspect the quality of the weld. |
| Intergranular | Rare, called Sensitisation. If Carbon level too high it combines with Chromium to form Chromium Carbide at approx 450-850 deg.C. Often occurs during welding | Three ways to combat: Anneal the stainless after it has been heated in this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming. When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel. Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be “stabilized” |
| Galvanic | Two different metals come into contact with each other and an electrolyte e.g. water producing a galvanic cell and causing a corrosive effect on the least ‘noble’ metal. When the current flows, material will be removed from one of the metals or alloys (the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected. | The rate at which corrosion takes place is determined by : The distance separating the metals on the galvanic series chart The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also. The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one. Separate the metals using a non-metalic insulator such as rubber |
| Micro Organisms | These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these “bugs”, one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The “bugs” fall into three categories:
| If the protective oxide layer is removed from stainless steel because of rubbing or damage, the “bugs” can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile. |
GALVANIC SERIES OF METALS AND ALLOYS
Corroded End ( Anodic or Least Noble)
MAGNESIUM
MAGNESIUM ALLOYS
ZINC
ALUMINUM 5052, 3004, 3003, 1100, 6053
CADMIUM
ALUMINUM 2117, 2017, 2024
MILD STEEL (1018), WROUGHT IRON
CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
CHROME IRON (ACTIVE)
STAINLESS STEEL, 430 SERIES (ACTIVE)
302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
NI – RESIST
316, 317, STAINLESS STEEL (ACTIVE)
CARPENTER 20CB-3 STAINLESS (ACTIVE)
ALUMINUM BRONZE (CA 687)
HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
LEAD-TIN SOLDERS
LEAD
TIN
INCONEL 600 (ACTIVE)
NICKEL (ACTIVE)
60 NI-15 CR (ACTIVE)
80 NI-20 CR (ACTIVE)
HASTELLOY B (ACTIVE)
BRASSES
COPPER (CA102)
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
SILICONE BRONZE
NICKEL SILVER
COPPER – NICKEL ALLOY 90-10
COPPER – NICKEL ALLOY 80-20
430 STAINLESS STEEL
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
MONEL 400, K500
SILVER SOLDER
NICKEL (PASSIVE)
60 NI- 15 CR (PASSIVE)
INCONEL 600 (PASSIVE)
80 NI- 20 CR (PASSIVE)
CHROME IRON (PASSIVE)
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
316, 317, STAINLESS STEEL (PASSIVE)
CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825
NICKEL – MOLYBDEUM – CHROMIUM – IRON ALLOY (PASSIVE)
SILVER
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
GRAPHITE
ZIRCONIUM
GOLD
PLATINUM
Protected End (Cathodic or Most Noble)