Except for the instrument operation training available from the manufacturers of the equipment (similar as for ICP, ICP/MS, GC/MS, etc.), or in-house (better say in-lab) training, I did not hear of any other way to gain any kind of credible certificate. Anyway, even if there is some course of that type out there, it would not necessarily make you an expert. Only after hundreds and hundreds of analyzed and interpreted ferrograms you may be able to gain respect from peers and customers alike and be regarded as an expert. This is simple my opinion based on the experience ad observations while working in labs.
For quite some time I had similar thoughts. For example, I have seen a title Lubrication Engineer used quite frequently. I became interested in possibly obtaining such title, but was not able to find any educational institution that offers such degree. To my understanding, this “title” could be obtained (or earned) only from within the organization one is employed at (e.g. oil companies, additives manufacturers, etc.).