Three Contributors to Plating Problems on Steel Parts

December 4, 2013

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.

Your plating problems are not beamed down from here...

Your plating problems are not beamed down from here…

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.

Inadequate Cleaning

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.

Story of the sky photo

Interactive zoomable high definition photo


3 Facts About Scale (Iron Oxide) Rust On Steel

August 2, 2011

On hot rolled bars to be cold drawn, the dark oxide surface is called scale.

What we see on the surface is "scale"- a combination of oxides of iron.

Scale is the name given to the oxides of iron that are formed on as wrought products as a result of mill operations (high temperature rolling or furnace treatment)

Rust is the commonly used term for iron oxide from weathering or corrosion.

Scale is

  • Hard
  • Brittle
  • High Coefficient of Friction

So we need to get it off the steel if you are to have any chance of keeping the tool edge sharp.

There are 3  oxides of iron:

Hematite   ( Fe2 O3)  has a microhardness of  ~ 1030 D.P.H., is red  in color, and is not soluble in acid.

Magnetite  (Fe3O4)  has a microhardness of  ~ 420-500 D.P.H., is black in color, and is not soluble in acid. 

Wustite  (FeO) has a microhardness of  ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.

Hematite and Magnetite make up the outer layers of the scale, and due to their composition, make up the larger mass of scale present. Due to their hardness and quantity they are the real dealbreakers for machining as they create tool edge wear.

One of the ways that Cold Finished Steel bars aid machinability is  by removing these hard abrasive oxides from the workpiece, so that they don’t destroy your tools and contaminate your cutting fluids.

Bar Coils