The causes of plating difficulties on parts manufactured from cold drawn steel bars are neither mysterious nor beamed on to the parts from the outer reaches of the galaxy. Plating difficulties invariably are related to three potential contributors: Inadequate cleaning, Insufficient stock removal, and Features of the part being plated.
The location of the plating problems on your parts gives you a key to determining the mechanism of failure
- If the plating problems are occurring on both original bar surfaces as well as on as machined surfaces, Inadequate Cleaning is likely the cause.
- If the plating difficulties are only on the portion of your parts that are original bar stock surface, Insufficient Stock Removal is the most likely cause of the problem.
- If the plating is fine everywhere else on the part except near a particular feature, Retention of contaminating fluid by a feature of the part is the likely cause.
I used the term inadequate to describe the situation where despite efforts to clean, some soil or contaminant remains, interfering with the plating. The cleaning method employed could just be insufficient to the task of cleaning- not enough time, agitation, etc.
Or it could be the incorrect cleaning process being used. Acidic cleaners do not remove oils or greases. In fact, we oil our metallic products like tools, firearms, and fishing reels to prevent them from being attacked by the acids in our skin and the environment. Alkaline cleaners are needed to remove oils and greasy residues from steel parts. Solvents can be used to remove the bulk of oily residues as well. If an insufficient or improper preclean is performed prior to plating, oils or oily residues can remain on the surfaces of the parts and mask or obstruct the deposition of the metallic plate.
If the plating problems are occurring on both original bar surfaces as well as on as machined surfaces, this is likely the cause.
Insufficient Stock Removal
When I first started out in the industry, most steel bars were acid pickled prior to cold drawing. In pickling, acid “wets” the entire bar surface, is able to penetrate through the surface scale to react with the acid soluble iron oxide known as Wustite, (FeO) on the innermost bar surface, and thus assures the nearly complete removal of all scale from the bar surface. (See scale note below)
Acid disposal became a significant operational challenge, and the industry moved to the use of mechanical descaling (shotblasters) to abrade away the hard iron oxide scale from the surface of the bars.
Unlike pickling, shotblasting does not fully remove every bit of scale- the shot stream abrades off most, but not every single bit of scale on a bar’s surface. If the bar surface has many fine depressions or pits, the abrasive shot may not be able to contact the scale at the bottom of these depressions. The presence of this scale could interfere with the subsequent plating of parts by the following mechanisms:
- It can retain metalworking fluids or cleaner from the precleaning step and then release these during plating causing localized reactions and staining;
- Because scale is an insulator, it will prevent electrical current flow at its location and thus mask or prevent the deposition and adhesion of the plate;
- It can create an air bubble by geometry as well as perhaps a hydrogen bubble if the bath is acidic. this bubble could form a mechanical barrier masking its location and preventing deposition/ adhesion of the plate.
If the plating difficulties are only on the portion of your parts that are original bar stock surface, this is the most likely cause of the problem.
Part Geometry Features and Location
Many times the design of the part can be the cause of the plating difficulties.
Features like small diameter holes, blind holes and recesses and grooves which can retain fluids, create bubbles or support a meniscus can result in localized contamination, create staining, and interference with deposition by providing a fluid or bubble barrier.
If the plating is fine everywhere else on the part except near a particular feature, retention of fluid by a feature of the part is the likely cause. Adding a wetting agent to reduce surface tension in cleaner or rinse can eliminate the problem.
There are other problems that can arise during plating that can be attributed to the plating process itself, but it has been my experience that these 3 categories will cover most of the problems encountered when both the machine shop and the plater claim that “there must be something wrong with the steel.”
If the machined surfaces plate fine- but not the original bar surface nor the inside of a hole- it isn’t the steel. It’s one of the above.
Scale Note: There are two additional iron oxides that could be present- Hematite (Fe2O3) and Magnetite (Fe3O4) – both of these are acid insoluble, but for this discussion, it is sufficient to say that they are removed by the removal of the underlying Wustite scale during pickling.