Environmentally and people-friendly staining. But how?

Manufacturers of mordants for the stainless steel processing industry do not like to mention on their labels that a product is toxic or environmentally unfriendly. With dry eyes, they advertise "non-toxic" and environmentally friendly pickling products. With an amendment to the CLP regulation in late 2024, this seems to have come to an end. The new regulation - valid throughout the EU - is thus a step in the right direction to ensure consumer safety and clarify the legal framework for manufacturers.

To ensure the corrosion resistance of stainless steel, a perfect surface is necessary. This must be free of run-on colors, boron emulsion residues, fly rust or ferritic damage after a casting process. The pickling products required for this purpose are based on nitric acid and hydrofluoric acid. In addition, there are ammonium bifluoride-based products that are often touted as being "non-toxic" and "environmentally friendly. But is this true?

Analysis of ammonium bifluoride

The substance ammonium bifluoride is a salt of hydrofluoric acid (HF) with the formula NH4HF2 and as a raw material is Acute Tox. 3 (H301, toxic if swallowed). Final verdict: toxic. But then, is this substance perhaps less dangerous than others? Unfortunately. Robbert Vermeule, owner of Sir John B.V. in Moordrecht: "When combined with acids such as nitric, phosphoric or sulfuric acid, dangerous decomposition products such as nitrous gases, hydrofluoric acid, nitrogen oxides (NOx) and ammonia are produced. So while these so-called "spin-off" products may not be present as such in the product, they are guaranteed to be created during use and must therefore be reported. And that does not happen everywhere."

New regulation

Since Dec. 10, however, an amendment to the CLP regulation has made this an EU-wide legal requirement. CLP stands for Classification, Labeling, Packaging and has been amended, among other things, with regard to the classification and declaration of hazardous mixtures. Vermeule: "In the context of pickling agents for stainless steel, two issues are important: the correct consideration of spin-off products - such as the aforementioned hydrofluoric acid - and the legal requirements for advertising and labeling hazardous mixtures." In terms of spin-off products, he refers to Article 14, which has been expanded to indicate that reaction products must also be included in the Safety Data Sheet classification; provided their formation is clear. And the latter is the case. Vermeule: "If hydrofluoric acid is not correctly listed as a cleavage product or reaction product on the safety data sheet, then the manufacturer is legally in violation." Moreover, the regulation goes so far as to apply not only to "legal" documents such as the label and safety data sheet, but also to advertising claims and even product names. Vermeule: "Terms such as 'non-toxic,' 'harmless' or 'environmentally friendly' are therefore not allowed. Not even when a mixture is exclusively classified as corrosive. After all, in that case the term 'non-toxic' would downplay the danger of 'corrosive.'"

Never environmentally friendly staining?

So there are no ways to pickle without harming people or the environment? "Of course there are!" laughs Vermeule. "You just have to know what you're doing. When you know that dangerous substances for people are created when used, you have to make sure you have adequate protective equipment and good working procedures but also: know what to do if people come in contact with it. As for the environment, it is not so much the mordants themselves that are the problem. Their acids are easy to neutralize and remove from wastewater. What is a problem is the heavy metals released by the pickling process. For example, iron, chromium and nickel. These must be disposed of responsibly with a wastewater installation."