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SHINY STUFF-

Chrome don’t get 'ya home, but...

by Jimmy Johnson, illustration by Anne Mitchell -

Reprinted with permission from Thunderpress January 1998 edition

Chrome brings visions of sparkling new parts and is not only used to improve the appearance of metals but to protect it from corrosion. To many people, plating conjures up images of parts being dipped in vats of molten metal or some kind of paint being sprayed on. What I will try to do is dismiss some of those myths about electroplating.

Electroplating is both a science and an art that requires an understanding of chemicals, metallurgy, electricity and how to combine all three to achieve the desired result. Electroplating is the process of putting a metallic coating on a metal (electrolysis) by passing an electrical current through a nonmetallic solution (electrolyte) containing a dissolved form of the metal to be plated.

Chemistry 101

Electrolysis is defined as: The chemical breakdown of an electrolyte (any substance which in solution form is capable of conducting electricity) by the action of an electrical current passing through it by means of conducting electrodes. These electrodes are called anodes and cathodes. Ions (atoms with a positive or negative electrical charge) move towards the opposite-charged electrode, where they give up their electric charges and become uncharged atoms, i.e., the chrome has stuck to the piece being plated!

Solid foundation

The first step in the process is preparation of the piece by removal of the grease, the seals, the gasket materials, the bearings (not bushings), bearing races and any and all paint. If these are removed before going to the stripping tank then the stripping acids do a better job and the chrome job turns out a whole lot better. Cleaning and stripping is one of the most im­portant steps in the plating process because the article’s final appearance depends primarily on a good solid foundation. If the piece is not clean then Three things can happen.

The piece will refuse to plate

The plating will be patchy or spotty

The plating will prematurely peel

Take it all off

There are different stripping procedures required for different base metals and for different types of previous metal coatings. Aluminum and steel, two of the most common on motorcycle parts, go through two different types of stripping processes. Aluminum, being softer, will suffer destruction when faced with the steel stripping process. Steel with paint on it can be dipped in a hot caustic soda, and aluminum with paint goes into an epoxy stripper. Old plating is removed by a reverse current sulfuric acid bath where the piece becomes anodic instead of cathodic. Stripping costs can add a substantial amount to the final bill. Today’s stronger-bonding epoxy paint is extremely hard to get off the base metal. Stripping, sandblasting and re-stripping are often required to remove undesired coatings.

Getting buffed up

In this order, these steps play a vital role in preparing the surface for a quality finish. Grinding involves removal of very large amounts of metal to get the general shape of the final product. Polishing is surface enhancement by means of metal removal which gets the article to the final shape. Buffing achieves the desired finish without much metal removal.

The base metal determines how coarse an abrasive is used for the surface enhancement operation. It has to be coarse enough to take out any imperfections, yet not too coarse because the rough wheel marks will have to be worked out. Each preced­ing step’s rough wheel marks must be taken off using less coarse wheels so the desired mirror-bright finish is achieved.

Porosity in the base metal is the cause of pitting, it’s like a sponge. This commonly affects older aluminum parts such as early FL sliders, early inner and outer primaries, early rocker boxes and Narrow Glide upper triple trees to name a few. There are ways to fix the pits but it can be very expensive and time consuming and doesn’t hold up as well on a street-driven bike. Pitting is some­times impossible to get out because the more it is polished, the bigger the pits become.

The next step in the process after polishing is cleaning and the base metal determines how the article is cleaned. Aluminum and steel are both dipped into an air-agitated, hot soap bath and scrubbed by hand to remove the heavy buffing compounds that are associated with polishing. Steel goes into a reverse current electro-cleaning tank to gas the oxygen off and remove any last traces of grease and dirt. Aluminum goes into a nitric “desmutting” acid chemical process and is coated with Zincate to aid in thor­oughly cleaning the base metal.

During the cleaning process, rinsing of both the aluminum and steel is paramount between each step to prevent carryover of chemicals from one tank to the other which could pollute and/or dilute the tank. The aluminum and steel process come together with the copper plating which gives a good corrosion inhibitor, aids in further cleaning and gives the nickel a better surface to adhere to. Keep in mind that this copper flash is only done to give the plater a “peekaboo” into how the piece will ultimately come out. If it ain’t right here, then the piece goes back through the cleaning process again to ensure a quality job.

Up to this point, most of the process has dealt with cleaning and preparing the metal for the nickel sulfate bath, a non-cleaning electrolyte, so it is essential that the piece be clean or else the nickel will not stick. Nickel is the “jacket” that accounts for most of the finish and brightness. The part is placed into the nickel bath and connected to the negative end (cathode) of a source of electricity. The positive electric terminal (anode) is connected to another conductor which is also placed into the bath. The electrical current separates the nickel in the bath and the coating is deposited on the metal surface of the article being plated.

The thickness depends on the strength of the current and the duration in the bath. Could be anywhere from a half a thousandth all the way to six thousandths of an inch. The amount of nickel applied will vary depending on the tolerances of a given piece. Not as much plating should go on an oil pump as a primary cover, because of the tighter tolerances needed on an oil pump.

Chrome it

After a thorough rinsing, the piece is ready for the chromium acid bath. Chromium, being an element, is chrome; but, just as cooks have different recipes, chrome processes come in varieties. One process being the hexavalent type, which is the brighter blue variety. The trivalent type process produces a gray-yellow variety of chrome. Trivalent is said to be more environmentally friendly, but hexavalent can be just as friendly to the environment if handled correctly and the proper final rinsing agents are used to convert hexavalent to trivalent. By the way, environmental concerns add a substantial amount to the final cost of the chrome.

Chrome is a low-efficient bath which means that it takes 36 times as much energy to throw (plate) chrome as it does to "throw” nickel. That amounts to about one amp per square inch. Sometimes a piece yellows in low-lying (low density) areas, which is the nickel showing through, and extra anodes are then placed on the piece to help the chrome get to it's proper destination. Some pieces require over 1000 amps to get the chrome to “throw down" in these low-density areas on the piece being plated.

After final rinsing, the piece is un-racked, wiped down and chrome buffed. Since so much amperage is used during the chroming process, areas of high density have accumulated some white spots that have to be buffed out. Chrome buffing also helps take out some of the imperfections and makes the surface smoother.

Double and triple plating

The terms double, triple, and quadruple plating are sometimes confused with different thicknesses of the deposit rather than how many coats of the different types of metal plating are on the piece (such as copper, nickel, and chrome) and not necessarily the thickness of the plating.