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Stainless is not always stainless - West Country Welding Supplies, UK - West Country Welding Supplies, UK

Stainless is not always stainless

The concept of "stainless" or "rust-free" steel is a qualified truth.

Common stainless steel will not usually rust when exposed to normal atmospheric conditions, but in other surroundings, e.g. in acids or in chloride containing water, the risk for corrosion increases. The fact that stainless does not rust in many situations does not mean that the metal itself is noble. The corrosion resistance is due to the steel surface reacting to oxygen, which results in the formation of a thin protective layer that prevents corrosion. The oxide layer, consisting mainly of chromium oxide, is called a passive layer, and the process of building the layer is called passivation. Corrosion of the stainless steel may occur when this thin, protective oxide layer is damaged or disturbed, or depleted in chromium, which often happens after exposure to high temperatures, for example during welding.

The need for postweld treatment of stainless steel welds

A protective oxide layer can be rebuilt if both the weld heat tint and the underlying chromium depleted layer is removed in the proper manner. The corrosion resistance of the stainless steel will then be restored.

Chemical treatment means that the damaged oxide layer is dissolved, usually with acid solutions. There are two main methods - pickling and electrolytic polishing. Mechanical treatment means that the surface layer is removed by grinding, abrasive blasting or brushing.

Pickling is the superior method

Research has been carried out to evaluate which method produces the best corrosion resistance. The comparisons show that pickling is the best method for achieving optimum corrosions resistance. Results of two sets of tests are shown below.

Requirements for successful pickling

The pickling agent needs to adhere well to the surface. Different pickling products are available on the market - pickling paste, pickling gel and pickling solution. Pickling paste is often to be preferred because it adheres best to the stainless surface, and neither runs nor falls off. Additionally, paste is more worker friendly in that the splash danger is minimised. The product should remove all heat tint, the underlying chromium depleted layer, as well as residual slag. The pickling agent should, under normal conditions, (15-22oC) give a clean surface after about an hour.

Go to Stainless Steel Pickling Products

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