Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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roughness, poor covering or throwing power, and will show dull deposits with slow plating speeds. If ripple current is suspected it is easiest to have a qualified elec- trician check the AC line balance of the incoming AC power source. They should also check to assure diodes are functioning. Today's rectifiers are controlled by electronic signals generated by a thyrister firing circuit. If these devices are defective they also can be a source of problems. (See the chapter on DC Power Supplies for more information.) SPECIAL CHROMIUM CHEMISTRIES Some applications for chromium require special or unique adjustments to the basic chemistry to give the quality needed for that application. Cast iron, for instance, has been a particular difficult substrate to plate. Alteration of the chrome bath chemistry along with preplate conditioning will allow for a satis- factory deposit with good adhesion. Some applications benefit from thin hard chrome deposits (also know as thin dense chrome) that can be used for bearing surfaces. These applications take advantage of the high lubricity of the chrome layer but with thin deposits that will not crack under the pressure of such applications. These thin layers of chrome also give some corrosion protection in lubricated conditions. Porous chromium had a wide use during WW II through its ability to develop channels for oil flow. This was excellent for propeller-driven aircraft. This is still used in some industries today, although the channels are created post plating. Crack-free chromium was used for many applications. The crack-free deposit is created by high temperature operation, with specific catalyst concentrations. The drive behind this is to develop a layer of chrome that will not have the net- work of cracks (micro or macro) typical of most hex chrome deposits. Based on the hardness of even the thinnest layer, cracking will occur. The application today is left only to very highly specialized applications. Black chrome is a deposit that had applications for solar panels due to its absorbance and low emissivity of light. Today it is used more as a decorative process. More and more applications are being used where inert anodes such as pla- tinized-titanium is utilized. There are tremendous benefits from such use of these anodes but the under laying problem is the creation of trivalent chromium. However, unlike processes that use lead anodes, the trivalent is not easily converted back to hexavalent. Some have tried to resolve this by adding chemical lead into the bath to form thin lead peroxide or the use of lead anode in conjunction with the inert anodes to help develop the lead film. These types of applications have problems as they try to maintain bath purity and deposit quality. Some chemistries are available that can help resolve this issue. IMPURITY REMOVAL Perhaps the major complication to chrome plating that makes a simple chemistry into a difficult control problem is the introduction of impurities. As stated before, even the chrome plating chemistry creates its own impurity: trivalent chrome. Under normal conditions this will be converted back to hexavalent chrome at the anode. The formation of trivalent is further complicated by poor anode (Pb alloy) maintenance. Typical lead anodes will be consumed and the dimensions change, thus limiting their effectiveness in oxidizing Cr (III) to Cr (VI). 194

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