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Thursday, October 23, 2008

All About Stainless Steel by Bob Campbell

There are over 1500 types of stainless steel. Each having it’s own unique quality. In our applications of electrolysis we are manly concerned with the non-corrosive qualities of stainless steel and therefore we have come to use either 304 or 316l stainless steel. The letter “l” designates a low level of carbon.

One of the most often asked questions is “does stainless steel contain iron, and why is it not magnetic?” The answer is yes. Stainless steel has an iron content of 58% - 69%. The magnetic permeability of the Alloys 316 and 317L in the annealed condition is generally less than 1.02 at 200 H (oersteds). It is essentually not magnetic because the crystalline properties of the alloy change the temperature at which the steel ceases to be magnetic. This temperature is called the Curie point. For example iron looses it ferromagnetic quality above 768°C. 300 series stainless steel is an alloy referred to as gamma phase iron or austenite stainless steel. There are however some stainless steels that are magnetic. These are referred to as martensite stainless steel.

One of the other concerns we should have is the electrical resistance of stainless steel 316 has 74.0 Microhm-cm. and 317 has 79 Microhm-cm. When used with high amperage this resistance can create a great deal of heat.Our application demands that we use austenite stainless steel because we want the iron to be locked into the crystalline structure of the alloy so that it will not corrode. When nickel or manganese is added, the austine structure of iron is stabilized and the crystalline structure binds the iron in the alloy and does not allow it to oxidize.

The 300 series of stainless steel contains a maximum of 0.15% carbon, a minimum of 16% chromium and enough nickel and/or manganese to retain an austenitic structure. The chromium forms an extremely thin, less than 0.0000001-inch thick, protective film of chromium oxide, which prevents further corrosion.

The next question is why do we condition our cells? The conditioning involves a process called passivation. This is not generally a process to be undertaken by laymen. It involves very dangerous chemicals.

The ASTM A380 describes passivation as "the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself." Further more the ASTM A380 goes on to say "the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film."

So there you have it. We want to do this to remove free iron left over from the machining process. Passivation is a cleaning process, which also provides a thin protective film.

The use of grinding wheels, sanding materials or wire brushes made of iron, iron oxide, steel, or zinc may contaminate the stainless-steel surface. Even the use of a grinding wheel that was previously used on other metals can cause contamination. If you sand blast your plates be sure to use iron-free silica or alumina sand. Stress relieving, annealing, drawing or other hot-forming processes can push contaminants deeper into surface and make it impossible to remove with passivation. Therefore I do not recommend sanding your plates.

Cleaning is done by first removing any oil, metal or organic material from the surface. For our purposes a solvent cleaning and soaking for 30 minutes in a 5% by weight solution of sodium hydroxide at 160 to 180°F will do the job. It is very important that this cleaning be done first otherwise the acid will react with the greases and prevent the passivation process.

Next rinse the plates and then soak the plates in a 10% by weight solution of citric acid for 30 at 150F. While nitric-acid-based solutions can be used, citric acid passivation is the preferred solution because it avoids the use of mineral acids, and the disposal problems associated with mineral acids. Citric acid is considered environmentally friendly. It is important not to use excessive bath temperature, or exceed the recommended immersion time. Also be careful with contamination. Citric acid is more prone to “flash attack” than nitric acid but in my opinion the safety issues outweigh this drawback and can be controlled by following these rules. Some citric acid products contain corrosion inhibitors and wetting that reportedly reduce flash attack.

Flash attack also occurs when the passitating solution is contaminated with high levels of chlorides. Chlorides in tap water are usually low enough. It is recommended that water with 50 ppm or less be used for rising but several hundred ppm can usually be tolerated. If you are concerned, you may wish to obtain an analysis from your water company.

Flash attack is evident by a heavily etched or darkened surface instead of the desired oxide film with a shiny, clean, corrosion-resisting surface that passivation is designed to optimize. Sodium dichromate can also be added to the rinse to reduce the chance of flash attack After Passivating the plates rinse and soak once more in a clean sodium hydroxide solution for 30 minutes to neutralize the acid then and dry. That’s it. Now your plates are ready. After all this work you will not want to touch those plates so be sure to wear clean gloves.

Bob Campbell

I love the smell of hho in the morning.


  1. Thanks Bob! Good information. This solved a few "Mysteries" for me.


    Chemical resistance properties of 317L and 316L

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