18-8 Stainless steel, 304, 316 Stainless Steel Corrosion

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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 the were installed. Everyday 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.

All metals except gold, platinum, and palladium corrode spontaneously

To understand the possibility of corrosion in stainless, we must first understand what gives it the ability to resist. Stainless steel is a family of alloy steels containing a minimum of 10-1/2% chromium. The chromium, when in contact with oxygen, forms a natural barrier of chromium oxide called a “passive film”. Only microns thick, this invisible and inert film is self repairing (according to worldstainless.org the chromium oxide film is 130 Angstroms  in thickness, an angstrom being one millionth of one centimetre) Alan Harrison, with the British Stainless Steel Association’s Stainless Steel Advisory Service, wrote us and advised he describes the thickness of the passivation as “about one ten thousandth of the thickness of a human hair”..

To ensure stainless steel is able to “self heal” itself, it is necessary that a finished product, i.e. fasteners, go through a process upon the completion of their manufacturing process. The process, called “passivation”, has become extremely controversial, and appears to have become less defined due to outside forces such as environmental regulations and high costs. The technical term and common usage are quite different. Technically, and still necessary to meet military and aerospace requirements, fasteners are submerged in a nitric acid solution. Also known as pickling, this acid treatment removes impurities from the manufacturing process, including oil and grease, and fine metal particles which have come from fastener tooling. Removing these exterior barriers or obstructions, the acid helps accelerate the formation of the chromium oxide film. In the US, the common use of this term among fastener manufacturers is simply a cleaning process. This can be done by different methods, from submerging in acid to dipping a finished product into a mix of cleaning fluid, and then leaving the fastener exposed to air. This “cleaning” can be fairly effective, or totally inadequate, depending upon the fastener manufacturer.

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.

1. 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.

2. 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.

3. 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.

4. 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.

The simplified galvanic series chart below will assist you in determining the potential electrical activity between 2 metals. Also included is a Guideline for Selection of Fasteners based on Galvanic Action