Quality Finishing: Troubleshooting Hexavalent Chromium Plating

Identifying the origin of a plating deficiency is the first step toward solving the problem. The source of poor plating can be divided into three categories: 1) Defective bath chemistry; 2) Improper temperature and/or current density; and 3) Poorly finished and/or cleaned substrates. An experienced plater can often identify the source of a problem without having to do much detective work; however, chemical analysis, Hull cell tests and reliable supplier recommendations are the best troubleshooting sources. The most common defects for hexavalent chromium plating deposits are listed here, along with some suggested solutions.

Despite the simplicity of their formulations, chromium plating baths are more complicated to operate than most plating baths and require more rigorous controls.

TROUBLESHOOTING HEXAVALENT CHROMIUM PLATING
Problem	Possible Cause Solution
Milky Deposit	High chromic acid/sulfuric ratio increase sulfate Chloride contamination remove chloride with silver carbonate Iron contamination dilute bath Excess trivalent chromium clean anodes; reoxidize trivalent chromium Poor nickel surface carbon treat nickel, improve rinsing of nickel, use nickel activator High temperature reduce to normal, check controller
Hazy Deposit	High chromic acid/sulfate ratio increase sulfate High chloride contamination remove chloride with silver carbonate Iron contamination dilute bath Excess trivalent chromium clean anodes; reoxidize trivalent chromium Low temperature increase to normal Current density too high adjust anode-to-cathode ratio Intermittent current flow check electrical contacts Poor nickel surface improve rinsing and use nickel activator
Gray, Dull but Smooth Deposit	High chromic acid/sulfate ratio increase sulfate Current surge when contact made reduce current High temperature check temperature controllers
Dull Spots in High-Current Density Areas	High chromic acid/sulfate ratio increase sulfate Passive nickel improve rinsing; use nickel activator Bipolar condition Use live entry to chromium bath
Blue Deposits	High temperature reduce temperature to normal
Rough Deposits	Low sulfate add sulfuric acid Low temperature adjust temperature to normal Surface preparation improve cleaning/rinsing Suspended particles in bath filter bath/eliminate source
Burned Deposits	High chromic acid/sulfate ratio increase sulfate Low chromic acid add chromium salts Excess trivalent chromium clean anodes/reoxidize trivalent chromium Too high current density reduce current density or increase temperature Low temperature increase temperature/preheat large, cold parts
Brown Spots or Rainbows	Low sulfate or catalyst increase sulfate concentration/submit samples for analysis Inefficient contacts check racks for buildup on hooks/tip, check for proper contacts on buss bars
Poor Adhesion	Insufficient etch increase etch time/check etch bath Surface contamination improve rinsing and/or cleaning cycle Intermittent contact clean/check contacts and put work in the chromium bath live Poor nickel deposit check surface prep before nickel plating and nickel bath
Poor Coverage	Low chromic acid content Add chromium salts Low chromic acid/sulfate ratio precipitate excess sulfate with barium carbonate Plating current too low raise current density Oxidized contact clean contacts Scaled anodes clean anodes High temperature reduce to normal Passive nickel activate nickel surface
Slow Deposition Rates	High chromic acid/sulfate ratio Add sulfuric acid Too low current density Increase voltage; check part distribution and for current leaks Scaled anodes clean anodes Oxidized contacts clean contacts Insufficient power supply increase rectifier size Iron contamination dilute bath Excess trivalent chromium follow procedure for reoxidation of trivalent chromium Too high temperature reduce to normal
Partial Deposition Rates	Too low current density increase voltage; clean rack contacts and anodes Uneven current density Improve part arrangement on racks Passive nickel activate cathodically or immerse in 50% hydrochloric acid Gas pockets Suspend parts so gas escapes
No Deposit	Reverse polarity make proper connections Defective contacts clean contacts Excess sulfate check ratio and correct Chloride contamination remove chloride with silver carbonate
Pitted Deposits	Pitted nickel deposit filter nickel bath Pitting in base metal Improve preparation Solution contamination from magnetic particles on ground surfaces improves grinding andcleaning procedure Gas pitting reposition work on racks