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