We have a 4140M component that was phosphated after machining. It was subsequently reworked and then phosphated again.
Now in certain areas there are surface markings on the machined surface. See enclosed picture. Its like something has been leached out? Note the groove has been emery papered by the workshop which is why it looks a bit polished. Has this been caused by "too much" phosphating? Or something else?
I didn't find the right solution from the internet.
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