Tank Chromium: Should I find an alternative?
By: John Langan, Ohio Contract Service Manager
Hard chromium for many years has enjoyed a reputation as being an electroplated deposit having many special properties serving several purposes. Chromium’s deposit properties include: hardness, low coefficient of friction, wear resistance, corrosion resistance, and oil retaining capabilities.
A common application for hard chromium is to rebuild or salvage worn parts. Examples such as rolls, journals, molding dies, combustion cylinders, and crankshaft journals are typical areas where hard chromium is deposited.
While chromium is a hard deposit, hardness and wear resistance are independent. Once the plating reaches .0008 inches or more, no difference is observed in wear resistance or hardness. On the other hand, chrome’s low coefficient of friction is directly responsible for its wear resistance, and is an important advantage. Another advantage of hard chromium plating is its ability to retain hardness throughout elevated temperatures.
Unfortunately, the environmental concerns surrounding chrome encourages users to find alternatives. The hexavalent compounds of the chromium plating process are both extremely toxic and hazardous.
Also, complications typical of the hard chromium bath plating process add to its disadvantages. Cathode efficiency is low. Conventional hard chromium baths have a cathode efficiency of 7-15%.
Because of the low cathode efficiency, hard chromium plating has a low deposition rate. Long plating times ranging from several hours to several days are necessary to achieve the proper thickness, creating lengthy lead times and production delays.
Hard chromium bath plating’s poor throwing power will also add to the production delays. Poor throwing power leads to insufficient deposit thickness when plating tight access areas such as into shoulders or blind holes. Parts are often plated with excess deposit to compensate, requiring post machining.
Because of the poor throwing power, extra consideration is needed for the design and use of plating racks and fixtures. The use of conforming anodes is critical in thick deposit applications to increase local current density in recessed areas. The shape, as well as the accessibility of the area to be plated can create complications when designing the conforming anodes. The positioning of the anodes in relation to the cathode may be restricted due to the parts geometry. Because of this, platers are at risk for uneven buildups or insufficient thicknesses.
While there are many reasons to switch from chrome tank plating, unfortunately, there is not one brush plating deposit that offers all of the properties of hard chromium. However, certain deposits have similar properties and are useful in salvage and repair applications, including a variety of nickels, sulfamate nickels, cobalt tungsten, and nickel tungsten. All of which performed similarly or better than hard chromium in terms of deposit quality, cohesion, and adhesion to the base material.
In summary, hard chromium plating cannot be entirely replaced using brush plating, however hard chromium usage can be reduced by carefully evaluating the application and determining if hard chromium is absolutely essential to your application.
Check back next week for our breakdown of the most suitable brush plating deposits to hard chromium.