engleski

Electrochemical optimization of environmental friendly stainless steel cleaning process

Chemical cleaning of stainless steel surface is often necessary step in the industrial production of metal parts due to the presence of particles of exogenous iron and iron compounds. Pollution of stainless steel with iron particles usually happens during the machining or forming process with steel tools. These iron particles disturb the passive layer and quickly form rust spots if subjected to corrosive environment resulting in loss of one of the most basic stainless steel characteristics, shiny corrosion-resistant surface. Cleaning treatment, in practice, is usually performed by using nitric acid based solutions according to the standard ASTM A967. Procedure of stainless steel cleaning with citric acid has been recently accepted, and introduced into the standards ASTM A967 and ASTM A380, as an alternative to environmental and health-risky treatment with nitric acid. However, due to the fact that procedure requires precise optimisation of process parameters, it is poorly accepted in industry. The type of stainless steel being processed is the determining factor when selecting the most effective cleaning procedure. The three major variables that also must be considered and controlled for the cleaning process are time, temperature, and concentration. A thorough knowledge of the material types and passivation processes is paramount to achieving the desired results. Improper bath and process selection and/or process control will produce unacceptable results, and in extreme cases, can lead to catastrophic failure, including extreme pitting, etching and/or total dissolution of the entire component. This work addresses the application of an environmental friendly process of cleaning stainless steel with citric acid. Process parameters have been optimized by electrochemical procedure in order to obtain the most effective cleaning and corrosion-resistant stainless steel surface. Electrochemical optimization enables precise determination of process parameters in comparison to the tests specified in standards that rely on the visual estimation of surface. Solution concentration, temperature, time of cleaning and passivation method were determined by electrochemical laboratory procedure for polluted and severely corroded 316L stainless steel samples. To verify the electrochemical results salt spray tests were performed on the real samples comprising of five different stainless steel materials exhibiting various corrosion resistance properties.

stainless steel; passivation; citric acid

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