Common substances that I find on stamped or machined parts are lubricants, cleaners and rust inhibitors. I do also sometimes find rust. But neither FTIR nor EDS can produce a firm diagnosis of “rust.” That’s because FTIR works best on carbon-based molecules, not ionic compounds (e.g., rust).
Rust is really a complicated material to analyze. It is an oxide of iron, but it has combined water molecules. The EDS will find iron and oxygen, but that’s the same things it will find for high-temperature iron-oxide scale, which is not loose, crumbly rust. Rust forms only in the presence of water/moisture.
Consequently, saying yes or no to the seemingly simple question “Is this rust?” is usually pretty difficult.
In many of the suspect reddish-looking contaminant projects, the red turns out to be most likely a discolored lubricant residue. The fact is that a cleaning process that does not remove all of the lubricant used to create the shape of the part is more likely to soak up cleaner, which also does not get rinsed away. Then if a rust inhibitor is applied, it simply covers up the residual glob of (often-degraded) lube and cleaner, which awaits the eyes of an inspector.
How do I know this? Well, 30 years of trying to figure it out for my clients who make small complex-shaped parts with stringent cleanliness requirements has led me to develop a process. But it’s often challenging. The clients who are able to bring PRODUCT data sheets, not just MSDS information, for every substance that is included in the process will help me figure it out. MSDS only require the toxic substances. Product data sheets often have info on the general product, even if it’s not toxic.
The clients who are able to bring samples – clean and used – of lubricants, cleaning solutions and rust inhibitors all increase the chances that I can help them figure out what the problem is. Having known reference specimens is a key to getting the best information out of any contamination study. Having a list of all processes in order experienced by the part is also very helpful.
Years ago, I found lots of sodium and chlorine on some stained parts. There was not supposed to be any of those substances in their stamping lube or cleaner. Then someone remembered that they brush soap on the parts as part of a 100% leak test. They hadn’t brought any of the soap, but I got a sample from the hand soap dispenser in my lab. What was it? Carbon, sodium and chlorine! Problem solved. That was not that long ago, and I was really surprised to find the same elements present in table salt in hand soap. I guess shampoo always has sodium lauryl sulfate. I'm still not sure where the chlorine comes from, however.