Pickling and passivation of stainless steel equipment
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There is a lot of information available online regarding the acid washing and passivation of stainless steel equipment, but the content is largely similar. Acid washing generally involves the use of a mixture of nitric acid and hydrofluoric acid. However, there is very little information on the specific requirements for controlling the actual processing steps. First and foremost, there are differences in the ways in which the concentration ratios are specified – some use volume ratios, others use mass fractions, and still others use mass/volume ratios – and when everything is converted to g/L, it becomes clear that there are significant variations among the different sources of information. For now, it is based on 380 as a reference; the relevant domestic standards are too outdated and inaccurate. Next is the control of the components in the solution; I searched for a long time but was unable to find a practical method for testing the concentration of hydrofluoric acid in mixed acids. In actual production, it was found that when the total acidity remained unchanged, the corrosion of stainless steel ceased. The amount of added hydrofluoric acid cannot be determined due to the lack of testing methods. Another issue is that of discarding the tank solution; it was initially planned to measure the iron ion concentration, just as is done in hydrochloric acid pickling, in order to determine whether the solution should be discarded, but no testing method for iron ions in nitric acid was found
These are all technical secrets, right? Moreover, different materials are used in different concentrations
The process procedure and operation manuals for certain companies that are uploaded online do not provide detailed information on process control; is this done for confidentiality reasons?
Well, what is the main purpose of pickling and passivation?
There is a stainless steel pickling and passivation technique from Europe that differs from the existing treatment methods in China. It operates on the principle of electrochemical reactions, using equipment in combination with acid solutions. These acid solutions do not contain highly corrosive substances such as nitric acid or hydrofluoric acid; their main component is citric acid monohydrate, which can come into direct contact with the skin without causing any harm to the human body； Basically, all types of stainless steel grades – 200, 300, 400, Duplex, etc. – can be processed. The treatment yields good results: the oil content after degreasing can be reduced to 14.5 mg/square meter, and the salt spray test duration after passivation exceeds 1200 hours. Complies with the requirements of American standards ASTM380 and A967.
I checked the U.S. patent for stainless steel passivation, United States Patent 3642586, and generally, the following conditions need to be controlled: 1, temperature； 2 ； Passivation solution pH value ； 3. Passivation time ； 4. Passivation solution ratio. Primarily targeted at 400 series stainless steels.
We hired various companies for the pickling and passivation process in our company, and no technical secrets were found
I tried using 50% nitric acid (by mass) for passivation, and the results were good; however, nitric acids with such a high concentration are no longer suitable for current production processes and conditions. No very good alternatives were found either
It seems that these ratios are determined by various companies based on their experience, with no clear standards