White Haze on Stainless Steel


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Q. Has anyone experienced a white haze forming on stainless steel metal surface after cleaning with n-propyl bromide (nPB) solvent? C.F.

A. I am not aware of a specific issue related to stainless steel and nPB. From purely a chemical standpoint, there is no reason that the nPB would react with stainless steel, so other things to consider are the lubricant and the state of the nPB. If the nPB is used and has built up contaminants over time, it is more likely to contain byproducts that could react with the oxide on the stainless steel. However, I think that is a relatively unlikely scenario.
What is more likely is the solvent reacting with the specific contaminant or lubricant you are trying to clean from the surface. In chemistry, the saying “like dissolves like” goes a long way in the area of industrial process cleaning. What this means in chemical terms is that chemicals of similar polarity dissolve into one another much easier than those of different polarity. Standing in two separate camps are water (very polar) and solvents (non-polar). In simple terms, that is why it is difficult for pure water to displace or clean a conventional oil from a surface. Detergents and other additives are required to lower the interfacial tension between the water phase and oil phase in order to suspend the oil in the water.
In a similar way, it is difficult for a solvent to dissolve or clean a water-based contaminant like a coolant. Like detergents, many coolants have additives that will change their properties to some extent, but in bulk, the fluid will essentially still behave like that of water. As a result, the solvent (nPB) can mix with the coolant, possibly extracting certain additives, and create an emulsion or mixture of the solvent and part of the coolant. This mixture may then be non-volatile and not evaporate readily from the surface, which could leave behind the haze you have observed. Readers with similar experiences, please email.