Quality Finishing: Troubleshooting Hexavalent Chromium Plating

Article From: Products Finishing,

Posted on: 2/1/2003

Even though many are not using hexavalent chromium baths or phasing them out of production, there are still finishers who use them and will continue to plate from them…

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

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