Resistance to Corrosion: Is Stainless Really "Stain-Less"?
A common misconception about stainless steel is that is not affected by corrosion. While misleading, the phenomenal success of the metal makes this common belief understandable.
One of New York City’s most impressive landmarks is the stainless steel clad peak of the Chrysler Building. Built in 1930 of 302 Stainless, a recent inspection revealed no signs of corrosion or loss of thickness.
The tallest manmade monument in the US, the St Louis Arch, is entirely clad in 304 stainless steel plates. Except for cleaning, the stainless exterior of this monument has required no corrosion maintenance.
Closer to home, housewives work in stainless steel sinks that shine as bright as the day they were installed. Every day the average American will come into contact with numerous examples of the success of stainless steel. And while the name correctly signifies the rust resistant properties of the metal, “stain-less” is not 100% “stain-proof” in certain applications.
Types of Stainless Corrosion
According to the DOD Technical Bulletin Corrosion Detection and Prevention there are 8 separate types of corrosion, with only a few having a major impact on stainless steel. Please be advised the descriptions below are extremely brief and written in laymen terms. Before acting on any particular application, qualified advice particular to such application should be obtained.
Uniform Attack – also known as general corrosion, this type of corrosion occurs when there is an overall breakdown of the passive film. The entire surface of the metal will show a uniform sponge like appearance. Halogens penetrate the passive film of stainless and allow corrosion to occur. These halogens are easily recognizable, because they end with “-ine”. Fluorine, chlorine, bromine, iodine and astatine are some of the most active.
Crevice Corrosion – this is a problem with stainless fasteners used in seawater applications, because of the low PH of salt water. Chlorides pit the passivated surface, where the low PH saltwater attacks the exposed metal. Lacking the oxygen to re-passivate, corrosion continues. As is signified by its name, this corrosion is most common in oxygen restricted crevices, such as under a bolt head.
Pitting – See Galvanic Corrosion. Stainless that had had its passivation penetrated in a small spot becomes an anodic, with the passivated part remaining a cathodic, causing a pit-type corrosion.
Galvanic Corrosion – Placing 2 dissimilar metals in a electrolyte produces an electrical current. A battery incorporates this simple philosophy in a controlled environment. The current flows from the anodic metal and towards the cathodic metal, and in the process slowly removes material from the anodic metal. Seawater makes a good electrolyte, and thus, galvanic corrosion is a common problem in this environment. 18-8 series stainless fasteners that work fine on fresh water boats, may experience accelerated galvanic corrosion in seawater boats, and thus it is suggested you examine 316 stainless.
Intergranular Corrosion – all austentic stainless steels contain a small amount of carbon. At extremely high temperature, such as welding, the carbon forces local chrome to form chromium carbide around it, thus starving adjacent areas of the chrome it needs for its own corrosion protection. When welding, it is recommended you consider low carbon stainless such as 304L or 316L.
Selective Leaching – Fluids will remove metal during a de-ionization or de- mineralization process. This usually happens inside a pipe and is rarely a fastener problem.
Erosion Corrosion – This corrosion happens when the velocity of an abrasive fluid removes the passivation from a stainless. Again, this is almost exclusively limited to pipe interiors and rarely a fastener problem.
Stress Corrosion – Also called stress corrosion cracking or chloride stress corrosion. Chlorides are probably the single biggest enemy of stainless steel. Next to water, chloride is the most common chemical found in nature. In most environments, the PPM are so small the effects on stainless are minute. But in extreme environments, such as indoor swimming pools, the effects can be extreme and potentially dangerous. If a stainless part is under tensile stress, the pitting mentioned above will deepen, and cracking may take place. If you are using stainless steel bolts under tensile stress, in an environment where chlorine corrosion is likely, you should examine the potential for stress corrosion cracking carefully.
According to a NACE International & CC Technologies study, corrosion costs the United States $276,000,000,000 annually. That’s $276 billion and 4.2% of the nation’s GNP. In the power generation and transmission industry alone, it is estimated that nearly 8% of the typical electric bill is attributed to the cost of corrosion.
It is unknown how many lives are lost annually due to corrosion but the number could be frighteningly high. Extreme examples through the years include collapsed bridges and
jet airliner crashes.
No metal, except for gold and platinum in their natural state are completely corrosion proof. Stainless steel has proven in thousands of applications that it is one of the most economical solutions to combat the ever present elements that cause corrosion.