Different environments present differing corrosive factors.  Near the ocean, environmental salt is a corrosive agent.  Near an oil well or refinery, sulfur is a corrosive agent.  In a chemical plant, acid is a corrosive agent.  There are many types of plating which will address these corrosive environments.

Plating thickness:  the more plating, generally the longer it takes for a corrosive agent to attack the underlying metal.  But, just applying more plating to a threaded piece, for example, changes the dimensions of the threads.  On long parts (i.e., 1/4″ X 6″ bolt), electro-plating will accumlate faster on the ends which could result in a .0002″ thickness in the middle of the part, but .00075 on the ends – including the threaded end.  This might make the part too large to put a nut on it.  In general, we must be aware of  the requirements for plating thickness so as not to cause problems for our customer’s applications.

Plating color:  We should be cautious about aesthetic specifications (example: “it must be brilliant”, or “it must be bright and uniform”).  We cannot apply subject standards to plating because one person’s “bright” may be another person’s “dull”).  In electro-finishing and dip-spin finishing we can apply a broad variety of colors. Sometimes, matching colors may be possible, particularly in the primary colors.

Corrosive resistance:  Different platings and plating thicknesses produce different corrosive responses.  The most common corrosive test applied to finishes is exposure to salt.  The process involves insertion of the plated part into a chamber in which a 5% concentrate of moist, salty fog is applied continuously to the plated part.  The observer notes how long it takes to produce white rust (typically, the first evidence of oxidation or corrosion), and then red rust (the oxidation of iron).  White rust shows the surface finish to be corroding, red rust shows the steel part underneath to be corroding.  Some examples follow that are more or less typical:

.0002″ hexavalent zinc  15 – 20 hours to red rust

.0002″ Trivalent (RoHS compliant) zinc with organic top coat - 90 hours to red rust.

.001 – .002″ Dip Spin painted finish with organic topcoat -  300 – 500 hours to red rust.

Dip-Spin and Electro finishes – dip-spin finishes typically start with either a zinc electro-finish which is then painted, or with a zinc phosphate undercoat which is then painted.  Typically, the paint is applied in a double-application w hich includes 1) application; 2) centrifuge to remove excess, then 3) baking to cure the paint.  Normally the dip-spin finishes are a minimum of .001″ thick but commonly .002″ thick or greater. 

Electro-finishes use water and electricity (electrolysis) to apply zinc to the surface of a part or parts, atom-by-atom.  The resulting thicknesses can be controlled very accurately, usually to the ten-thousandth of an inch (.0001″).  Electro-nickel is also applied atom-by-atom but when examined in an electron-microscope have the appearance of layers, whereas zinc is applied in a particulate fashion.  Layers are atomically weaker than particle accumulations which can result in flaking.  If you dump a box of nickel plated screws on the table, you will observe flecks of nickel on the table.

Dip-spin parts are very good against corrosion by comparison to most electro-finished parts, but the appearance of the dip-spin part is not as aesthetically appealing.  Dip-spin parts tend to have no lustre and sometimes have a coarser texture.   

There are all types of more exotic finishes, each of which is designed to produce a certain result whether that result be corrosion resistance, electric resistance (tin for example) hardness (hard chrome) or color (paint). 

Paint – In addition to dip-spin finishes which are primarily done for corrosion resistance, some consumers want heads that are painted to a certain color.  This is most commonly done so that the screw head matches the color of the plastic or aluminum into which it will be inserted.  But, color may also be for identification of some parts from others, or for color and corrosion.  Some paints will provide  up to 10,000 hours salt spray whereas some provide almost no corrosion protection.

Hydrogen embrittlement  is a common phenomenon and very dangerous.  Typically H2 embrittlement occurs when a concentration of hydrogen (the plating bath, or even from the air)  is introduced to a concentration of carbon which has been hardened (as in a heat treated fastener).  We normally do not expect H2 embrittlement to occur in low carbon steel fasteners which have not been hardened by heat and quenching.  But, the higher the carbon and the higher and longer the application of heat, and the fastener the quench, the more likely H2 embrittlement will become a problem when the part is electro-plated. 

In the electro-plating solutions is water and hydrogen.  Hydrogen is released when electricity flows through the solution.  If there is a heat treated fastener being electro-plated, the parts should be baked at 375 degrees F within 3 hours of plating.  The sooner after plating, the better.

When heat treated fasteners are hardened and quenched, small microscopic fissures appear in the atomic structure of the fastener.  These fissures are resting places for hydrogen.   If hydrogen is present, it will find and fill these fissures.  Over a short period of time, these fissures will become geometrically fixed in place.  The hydrogen will eventually depart but the fissures will remain in place as permanent weak spots in the steel.

If the plater bakes a part after hydrogen is introduced, the baking will “chase” the hydrogen out of the fissures which will then close up and restore a stable network of carbon and iron attoms in the steel fastener.  No fissures means high strength.  Presence of fissures which is caused by hydrogen  habitation is going to be a problem. 

Baking after plating is inexpensive and should be done anytime the core hardness of the fastener is Rockwell C35 or higher.  There is some debate in industry that has existed for 50 years about the trigger hardness.  Some say bake above RC28.  Others say above RC45.  I say, above RC35. 

Since the presence of H2 is also a causitive issue for H2 embrittlement, remember that if you have a choice, select a non-electro finish if possible, and remember that the thicker you want your plating the longer the parts remain in the plating bath.  So, more thickness increases the likelihood of H2 embrittlement.

For Sonfast Corporation, we bake whenever:

1) Case hard or heat treated parts are stripped and replated;

2) We plate a part that has a known history of embrittlement potential;

3) We plate a part that has a core hardness value of RC 28 and above including grade five or M8 parts, and higher.

By following theses strict guidelines, and asking our suppliers to follow these guidelines, we have avoided H2 embrittlement problems altogether for the last fifteen years.

Rich