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TROUBLESHOOTING METAL FINISHING PROCESSES

Logical analysis of the situation

This may not solve your actual problem, but it could tell you where you have to home in. So ask yourself some questions!

• How long has the problem been going on for ?
• Did it start suddenly or has it been slowly getting worse ?
• If it started suddenly, can you link it to any other event - like using a new supplier of chemicals, or a new piece of equipment, or a new guy working on the line or anything else that happened in the shop ?
• Is it possible the problem has been going on for some time but that you have only recently become aware of it ?
• Is the problem specific to just one finishing line ? If you have two or more similar processes, e.g two zinc plating lines, are both or all affected ?

If the problem has struck suddenly, then the odds are that something has changed, which causes the trouble. If it has been getting gradually worse, that's often a sign of a process solution slowly drifting out of 'spec.

Is the problem apparent pretty well the moment the work comes off the line, or does the problem appear after processed parts have been in store for a week or so ? In the latter case, the problem is almost certainly diffusion. This could be diffusion of a liquid or diffusion of a solid. Diffusion of a liquid - process solutions are trapped in cracks or pores and slowly diffuse out, reacting with one or more of the overlying coatings. Any type of casting or diecasting is especially prone to this. Use metallographic sectioning or other means to determine whether defects visible on the surface are associated with pores, blow-holes, cracks or other defects capable of retaining process solution. Remedy - better rinsing. Defects of this kind are usually localised, as one would expect. A quite different type of defect is widespread staining, e.g patches or patchy areas. These are much larger than could be associated with a pore. A common cause is solid state diffusion. Atoms of one metal are diffusing up to the outermost surface and causing a discoloration or staining. The answer here is to lay down a so-called diffusion barrier which prevents the metal atoms from diffusing out to the surface.

Maybe nothing has changed in your Shop. But are you processing work from
outside ? Has anything changed there ? Some issues to check out include:

• new supplier of metals or alloys
• change in cleaning procedures
• change in abrasive materials or abrasive processes used
• change in machining or cooling & cutting oils, change in use of short-term rust preventive coating
• use of new mold release agents, drawing lubricants etc