AD-A250 610 Report No. NADC-91114-60 NONCYANIDE CADMIUM PLATING BATHS F. Pearlsteln and and V. S. Agarwala Air Vehicle and Crew System Technology Department (Code 6062) NAVAL AIR DEVELOPMENT CENTER Warminster, PA 18974-5000 OCTOBER 1991 DTI ELECTR MAY 1919 FINAL REPORT Approved for Public Release; Distribution Is Unlimited. Prepared for Air Vehicle and Crew System Technology Department (Code 60) NAVAL AIR DEVELOPMENT CENTER Warminster, PA 18974-5000 92-13316 92 5 18 134.i
Transcript
AD-A250 610
Report No. NADC-91114-60
NONCYANIDE CADMIUM PLATING BATHS
F. Pearlsteln and and V. S. AgarwalaAir Vehicle and Crew System Technology Department (Code 6062)NAVAL AIR DEVELOPMENT CENTERWarminster, PA 18974-5000
OCTOBER 1991 DTIELECTR
MAY 1919
FINAL REPORT
Approved for Public Release; Distribution Is Unlimited.
Prepared forAir Vehicle and Crew System Technology Department (Code 60)NAVAL AIR DEVELOPMENT CENTERWarminster, PA 18974-5000 92-13316
92 5 18 134.i
NOTICES
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CODE OFFICE OR DEPARTMENT
00 Commander, Naval Air Development Center
01 Technical Director, Naval Air Development Center
05 Computer Department
10 AntiSubmarine Warfare Systems Department
20 Tactical Air Systems Department
30 Warfare Systems Analysis Department
40 Communication Navigation Technology Department
50 Mission Avionics Technology Department
60 Air Vehicle & Crew Systems Technology Department
70 Systems & Software Technology Department
80 Engineering Support Group
90 Test & Evaluation Group
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED
4. TITLE AND SUBTITLE S. FUNDING NUMBERS
NONCYANIDE CADMIUM PLATING BATHS
6. AUTHOR(S)
* F. Peaftein and V. S. Agarwala
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) B. PERFORMING ORGANIZATION* REPORT NUMBER
Air Vehicle and Crew Systemns Technology Department(Code 6062) NADC-91114-60NAVAL AIR DEVELOPMENT CENTERWarminster, PA 18974-5000
9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING I MONITORINGAGENCY REPORT NUMBER
NAVAL CIVIL ENGINEERING LABORATORY('ode L 74A,,r Hueneme, CA 9304Q-5000
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTION! AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE
Approved for Public Release: Distribution Is Unlimited
13. ABSTRACT (Maximum 200 words)
One approach to minimizing toxic wastes is to eliminate the use of cyanide plating baths. Non-cyanide zincplating baths have been successfully developed and have found widespread use. An Investigation wasconducted In an attempt to accomplish similar results with cadmium plating baths. The focus of this studywas on additives to a near neutral cadmium bath, free of complexing agents. A Hul cell was used to enablevisualization of deposits over a broad range of cathode current densities. Experimntal design (TaguchiMethod) was used to optimize bath parameters and constituent concentrations. Bath have been developedwhich Indicate promise for producing dense deposits with good covering power, and relatively low tendency
1. Graphical Representation of Hull Cell Cadmium Deposits Obtainedat 0.5 A for 5 Minutes in Baths of pH 6.1 at 270 C: (a) Base BathComposites; (b) Base Bath + 0.1 g/l MTA; and (c) Base Bath + 0.1g/1 MTA +0.08 g DCbPyl ...................................... 15
2. Structural representation of Dicyanobis 2,2'- Bipyridine Iron 11 .............. 16
NADC-91114-60
LIST OF TABLES
Table Title Page
I Composition of Experimental Baths Using Taguchi Experimental Design ....... 5
U1 Hull Cell Deposits from Experimental Baths with Numerical ScoresAssigned Based upon Deposit Appearance and Sound-DepositCovering Power ................................................... 6
III Total and Average Scores for Four Baths Containing Low andHigh Bath Variables ............................................... 7
IV Effect of Low and High Values of Bath Variables on PercentImprovement in Deposit Appearance ................................... 7
V Effect of Low and High Values of Bath Variable on Percentin Sound-Deposit Covering Power ..................................... 8
VI Cathode Current Efficiency of Proprietary PAVCO AmeribriteNeutra-Cadmium Bath at Various Cathode Current Densities ................ 11
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ACKNOWLEDGMENT
The spectrophotometric analysis work of Christine N. Dickeyis gratefully acknowledged by the authors. The financialsupport by the Naval Civil Engineering Laboratory is highlyappreciated.
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1. BACKGROUND
For many years, cadmium has been used by the Navy for coatingof steels to provide the following:
a. corrosion resistance in naval environmentsb. sacrificial protection to steel substratesc. galvanic compatibility with aluminum alloy aircraft
structuresd. favorable torque-tension characteristics of plated
aircraft fasteners.
However, owing to the toxic nature of cadmium and the adverseenvironmental impact associated with deposition and service,alternatives to cadmium have been explored.
1.1 ALTERNATIVES TO CADMIUM
Ion vapor deposited (IVD) aluminum [1] has found applicationsto naval aircraft, particularly the F/A-18 by McDonnell-Douglas Corp. IVD aluminum has found an important nichewhere electrodeposited cadmium is unsuitable, e.g., whentemperatures in excess of 4500 C may be encountered, whencontact of plated parts with titanium is expected (cadmiumcan cause embrittlement of titanium) and when cathodicprocesses are precluded for very high strength steels.However, the IVD process falls short as a viable universalalternative to cadmium because frictional properties of thealuminum are inferior and the process is quite expensive andnot readily available. Zinc deposits are approximately equal(or superior) in corrosion resistance to cadmium duringoutdoor industrial environmental exposure but cadmium is farsuperior at the marine environments that comprise major Navyinterest. In addition, zinc has the characteristic of formingvoluminous corrosion products which can result in seriousbinding of close-fitting parts. However, for many commercialand military applications, zinc is a viable alternative tocadmium when applied at approximately double the thicknesssuitable for cadmium. Zinc - 10 to 15% nickel alloyelectrodeposits (2] offer corrosion resistance superior tocadmium in most environments but the possibility ofdezincification, and leaving a nickel rich surface, wouldpreclude its use on components in contact with aluminumalloys because of adverse galvanic effects. The tin-20 to 25%zinc alloy electrodeposits are produced from cyanide-basedbaths (3); they offer good corrosion resistance but havefound little commercial interest over the years. It istherefore concluded that, at this time, continued utilizationof cadmium coatings is necessary for many Navy applications.
1.2 NONCYANIDE CADMIUM PLATING BATHS
Cadmium is readily and effectively plated from cyanide-basedbaths which offer ease of control, excellent throwing power
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and ability to produce dense, fine-grained or bright deposits.However, the cyanide baths present a serious health hazard toplating personnel when proper safety measures are not heeded;nadvertent mixing of acids with cyanide baths releasesdeadly hydrocyanic acid vapors. For this reason, there hasbeen a governmental initiative established to replace cyanidebaths used by the plating and surface finishing industry.The cost of waste treatment of cyanide is an additionalfactor to consider.
1.2.1 Acid Fluoborate
Acid fluoborate-cadmium plating baths have been widely usedby the military, in place of cyanide baths, to help alleviatehydrogen embrittlement problems as the acid bath has aconsiderably lower tendency to embrittle plated high strengthsteels. However, the bath is highly corrosive and poor inthrowing and/or covering power. The fluoborate bath cannot beconsidered as an alternative to cyanide baths for mostapplications.
1.2.2 Mechanical Plating
Mechanical (peen) plating of cadmium is accomplished bytumbling parts with glass beads in a special solution withthe calculated amount of cadmium powder added to provide thedesired thickness. The deposit is formed by the sliding andtumbling actions of the glass beads. Recent studiesindicated that mechanically plated zinc - 25% cadmium wassuperior to 100% cadmium in corrosion resistance and hadsatisfactory torque-tension drive characteristics (4]. Thisprocess produces little or no embrittlement to high strengthsteels; however, the process, while well suited to batchtreatment of small parts, is usually not appropriate forcoating large or heavy objects.
1.2.3 Proprietary Baths
Three proprietary noncyanide cadmium plating bath (two acid-sulfate and one alkaline ammonium sulfate-chloride ) werestudied by Boeing Aircraft Corp [5]. One of the acid baths"was ruled out because of poor appearance and cathodeefficiency." The other acid bath, pH @ 0.5, had someperformance deficiencies as plated parts were smutty and thecadmium content rose owing to anode dissolution during bathinactivity. The alkaline bath (pH 7 to 9) performed well ontests covering 59 ampere hours using a seven liter bath.However, the high concentration of ammonium salts in the bath(>1.5 molar) prevented precipitation of cadmium and othermetals that may enter the waste treatment process; thus,ammonium ions must be decomposed, usually by chlorination.Alkaline cadmium plating baths, based upon EDTA complexation,have been reported but EDTA complexes present essentially thesame order of waste treatment problems as ammonium compounds.
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It was considered that the most useful cadmium baths for Navyoperations would be near-neutral (slightly acidic) andessentially free of complexing agents. This report concernsthe efforts to achieve this goal.
2. EXPERIMENTAL WORK MQ RESULTS
2.1 SELECTION OF INITIAL BATH - HULL CELL TESTS
The main bath composition was selected based on theconsiderations of essential ingredients, and prepared fromanalyzed reagent grade chemicals:
80 g/1 CdSO.8/3H2040 g/1 Na S0410 g/l Na 2H O2
Plating tests were conaucted on polished steel panels using a267 ml Hull Cell with reciprocati'g paddle agitation. TheHull Cell [6] is a miniature plating unit designed to producemetal deposits over a wide range of cathode current density(CCD); the current density at any point along the cathode ispredicted by the equation (1)
i = I (27.7 - 48.7 log L) (1)
where i is CCD in A/ft 2 (mA/cm2 ). I is the total currentapplied in amperes and L is distance in inches along thecathode from the high current density edge. Thisrelationship is generally valid, though there can bevariations in specific baths where cathode polarization isparticularly high. The electrodeposited metal coatings wereevaluated for their appearance, covering power and "burning."For screening tests, 0.5 A were applied for five minutes withbaths at pH 5.8 + 0.1 and temperature 31 + 10 C.
2.1.1 Bath Additives
The simple bath as described above, produced little or nodeposit at less than 3 mA/cm2 , thin and highly porous depositat less than 25 mA/cm and 9ense matte deposit only atgreater than about 25 mA/cm . Variations in bathconcentration or substitution of potassium for sodium,chloride for sulfate or borate for acetate did little toimprove deposits. Consideration was then given to applyingadditives which are known to have beneficial effects inplating baths of various types. Sulfur-containing compounds,as a class, had the most beneficial effects on improvingcadmium deposit characteristics. Of these, 2-mercapto-thiazoline (MTA) was the most effective and at 0.1 g/l,dense, white watte deposits were produced at CCD greater thanabout 1 mA/cmi. At lower CCD, deposits were thin and poiousand did not extend below about 0.2 mA/cm'.
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Other additives that showed some beneficial effects werethiourea, diethylthiourea, sulfonated mercaptobenzothiazole,1,10-phenanthroline-iron, cetyltrimethylammonium bromide,hydroxyethylcellulose, 1,2-dimethylimidazole or 1-allyl-2-thiourea (ATU).
Since deposits from the bath containing 0.1 g/1 MTA wereconsidered inadequate in covering power for practical use,the effect of additional agents was considered for enhancingthis characteristic as well as for improving deposit .ouster.Agents which showed some promise in these regards are: 1,10-phenanthroline-iron, dicyanobis 1,10-phenanthroline iron II(DCPhI), dicyanobis 2,2'-bipyridine iron II (DCbPyI),polyethylene glycols, polyethylene oxides, quaternaryammonium chloride compounds, sulfonated mercaptobenzothiazole(SMBT). Interestingly, agents which improved covering power,usually also improved deposit lustre. Of these, DCPhI,DCbPyI or SMBT was most effective for improved covering powerand with potential for yielding semi-bright to brightdeposits. DCPhI or DCbPyI was considered most "romising andbehaved similarly but the latter was selected for additionaltesting as it was substantially more soluble. Approximately0.04 g/l DCbPyI yielded good results and addition of 0.3 g/1ATU was beneficial for improvinq appearance and reducing thetendency for formation of dendrites/nodules at the high CCDedge. A bath formulation showing promise was comprised asfollows:
80 g/l CdS0.8/3H2040 g/l Na S -10 g/l Na H 020.08 g/l 2-Rercaptothiazoline (MTA)0.04 g/l Dicyanobis 2,2'-bipyridine Iron II (DCbPyI)0.3 g/l Allyl-2-thiourea (ATU)
pH 5.8; Temperature 310 C
2.2 TAGUCHI METHOD OF EXPERIMENTAL DESIGN
It was decided to subject this formulation to the TaguchiMethod of Experimental Design [7,8] which is effective forscreening a large number of variables with minimum number ofexperiments. The LB (27) design matrix was selected todetermine the effects of plating bath variables, each at twolevels (higher or lower) from the initial bath compositionshown above. Table 1 shows the eight experiments (baths)when the selected six variables were changed in accordancewith the Taguchi method. It is apparent that with sixvariables, each at two levels, a total number of 26 or 64experiments would be required to cover all possible testcombinations. The conductive salt (sodium sulfate) and thebuffer (sodium acetate) were kept at constant concentrationfor these tests. In the case of MTA, the high value was notincreased above 0.1 g/l as the agent was difficultly solubleabove this level. It was found that ultrasonic agitationhelped speed dissolution of the compound.
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Table I - Composition of Experimental Bathq Usingthe Taguchi Experimental Design L8 (2")
Concentration, g/1Bath----------------------------------------No. DCbPyI ATU MTA CdSO4 .8/3H 20 pH Temp., °C
The eight formulations were allowed to stand for one weekprior to conducting Hull Cell tests, as described above, with0.5 amperes applied for five minutes.
2.2.1 Criteria for Evaluation of Deposits
DeRosit Apearance. Deposit appearance wa5 subjectivelyevaluated over the CCD range 4 to 20 mA/cm (roughly between1.5 to 6.5 cm from the high CCD edge). Numerical values werearbitrarily assigned as follow:
AssignedDeposit Appearance Numerical ValueBright - 100Semi-bright to bright - 80Semi-bright - 60Matte with some reflectivity - 40Matte, nonlustrous - 20
Covering Power. Covering power (CP) is normally determinedby the lowest CCD at which deposit forms. However, in someinstances, deposits at low CCD were extremely thin andporous. It was therefore decided to neglect these veryporous deposits as unacceptable quality and instead todetermine the lowest CCD to which "sound" nonporous depositsare judged to extend; this criteria will be called "sound-deposit" covering power. Numerical values were assigned asfollow:
Extent of Sound AssignedDeposit from NumericalLow CCD End. mm Value
0-1 100>1-2 80>2-4 60>4-8 40>8-16 20>16 0
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2.2.2 Evaluation of Hull Cell Panels
Based upon the criteria established above, the plated HullCell cathodes from each of the eight experimental baths wereevaluated and numerical scores assigned as shown in Table II.
Table II - Hull Cell Deposits from Experimental Bathswith Numerical Scores Assigned Based upon Deposit
It is evident that the deposit from Bath #6 was best inappearance and sound-deposit CP. It should also be notedthat dendrites were produced at the high CCD edge from Bath#1 (0.5 mm long dendrites) and Bath #7 (4mm long dendrites).After the eight experiments were run and the appearance andcovering power evaluations were compiled, the data weresubjected to analysis by the Taguchi method. The experimentaldesign array is orthogonal, or balanced, which allows for thecalculation of the effect of each variable independently.For any pair of columns, all combinations of levels willoccur an equal number of times. As a result, it is possibleto compare the average for the results of the high setting ofany variable to its low setting; the effects of all of theother variables will cancel out.
The sum of the above numerical scores was determined for thefour baths representing a low value of bath variable and thefour containing a high value of bath variable. For example,low DCbPyI concentration was in Baths #1, 2, 3 and 4 for atotal score of 140; high DCdPy I in Baths #5, 6, 7 and 8 fora total score of 190. Each total was divided by four toyield an average score of 35 and 48 respectively. Thesescores were then compared to the overall average; namely, 41.Likewise, appearance scores for low ATU concentration wereobtained from deposits of Baths #1, 3, 5 and 7; high ATU fromBaths # 2, 4, 6 and 8. The results were 170 and 160 withaverages at 43 and 40 respectively. The results for all bathvariables calculated in this way are shown in Table III.
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Table III - Total and Average Scores for Four BathsContaining Low or High Bath Variable.
Deposit A pearance Sound-Dep. Covering PowerBath Low Bath Var jih Bath Var Low Bath Var h Bath VarVariable Total Avg Total Avg Total Avg Total Avg
The average appearance and sound-deposit CP scores for eachbath variable (high or low) was compared to the overallaverage scores of 41 for appearance and 25 for sound-depositCP. The degree of improvement in % over the average overallscore effected by either high or low variable value is shownin Tables IV and V, for ease of evaluating the results.
Table IV - Effect of Low and High Values of Bath Variableson Percent improvement in Deposit Appearance.
Bath Low High % Improvement inVariable Value Value Deposit Appearance
It can be seen from Table IV that deposit appearance isaffected most strongly by the higher value of pH andmoderately by the higher value of DCbPyI and MTA. From TableV, sound-deposit CP was most strongly affected by higher pH,higher DCbPyI and lower temperature; moderate improvementresulted from higher MTA or CdSO4 concentration. Beneficialeffects on both appearance and sound-deposit CP resulted fromthe higher values of DCbPyI, MTA, pH and from the lowertemperature. ATU had little or no effect on the depositcriteria. The higher concentration of CdSO 4 improved sound-deposit CP somewhat but the lower concentration improved thedeposit appearance. It was considered more beneficial tokeep the cadmium content at the lower value to reduce thequantities dragged out during plating which would requirewaste treatment.
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Hull Cell tests were conducted on a bath containing 0.1 g/lMTA and various concentrations of DCbPyI; deposits improvedin appearance and CP up to 0.12 g/l though improvement wasonly marginal between 0.08 and 0.12 g/l. Increasing the MTAlevel from 0.1 to 0.2 g/1 in the bath containing 0.12 g/1DCbPyI, somewhat improved deposit appearance but not CP.
Table V - Effect of Low and High Values of Bath Variableson Percent Improvement in Sound-Deposit Covering Power.
Bath Low High % Improvement inVariable Value Value Sound-Deposit CP
Based upon these results, the following bath was consideredan appropriate compromise for providing good depositappearance and reasonable CP while minimizing the chemicaladditives used:
60 g/l CdSO4 .8/3 H2040 g/l Na SO410 g/1 Na ?H3020.1 g!lL M A
0.08 g/l DCbPyI0.15 g/l ATJ
pH 6.3; Ter, 270C
Figure 1(a) shows, diagrammatically, Hull Cell deposit fromthe above bath without MTA and DCbPyI. Addition of 0.1 g/lMTA greatly improved deposit characteristics but CP was poor;see Figure 1(b). Addition of 0.08 g/l DCbPyI, along with theMTA, resulted in semi-bright deposit over a wide CCD range.See Figure 1(c). The Hull Cell deposit of Figure 1(c) wassuperior in both appearance and covering power to that of anyof the original eight bath combinations of Table I,indicating the value of the Taguchi method for optimizingcompositions.
2.3 DECOMPOSITION OF DICYANOBIS
Three weeks after make-up of the eight Taguchi design baths,it was noted that the amber color of the baths fromdissolution of the DCbPyI was noticeably lighter than whenfreshly prepared. Decreased DCbPyI in the cadmium platingbath was confirmed by spectrophotometric analysis at 500 nmwavelength. The final bath formulation was then preparedwith and without cadmium salts. Various agents were added to
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samples of the complete bath to ascertain whether the rate ofDCbPyI decomposition could be reduced substantially; theagents were 0.5 g/l K4 Fe(CN)6 , 2-2' bipyridine, Na SO3 orEDTA. A bath sample was deaerated by bubbling wits nitrogenand another sample stored in the dark. All samples weresealed during storage and samples withdrawn periodically forspectrophotometric analysis.
In the absence of cadmium salts, but containing all otherbath constituents, the peak for detection of DCbPyI occurredat 520 nm vice 500 nm for the complete bath. It is thusevident that the cadmium salts interact with the DCbPyI. Ashift in wavelength in the direction observed is to beexpected in conformance with ligand field theory. The bathwithout cadmium salts showed no visual loss in color orreduction of absorbance value after standing 74 days. Evenafter many months, no perceptible change in color wasevident. On the other hand, when cadmium salts were alsopresent in the bath, the peak for DCbPyI showed gradual lossof color with time and decrease in peak absorbance value at500 nm with time. Based upon peak absorbance values, thefollowing approximate reduction of DCbPyI was indicated:
Storage Time, days 0 14 49 74% Reduction DCbPyI 0 37 70 86
Of the additives, only 2-2' Bipyridine (bPy) had asignificant effect on DCbPyI decomposition rate at up to 49days storage but had little benefit with longer storagetimes. The results are shown below:
Storage Time, days 0 14 49 74% Reduction DCbPyI 0 19 45 81(0.5 g/l bPy added)
Even so, addition of bipyridine reduced deposit quality andparticularly increased high CCD "burning". Aged baths,largely depleted of DCbPyI, lost ability to produce lustrousdeposits. Replenishment of DCbPyI restored deposit lusterthough there was somewhat increased tendency for "burning" athigh CCD, presumably from presence of decomposition products.Neither deaeration with nitrogen nor storage in the dark hadany significant effect on DCbPyI decomposition rate. It wasfound that the freshly prepared final bath could bedecolorized rapidly and completely by heating at 90 to 95 Cwith the same apparent results as long-term aging.
When dicyanobis 1,10-phenanthroline iron II (DCPhI) wasdissolved in the Cd bath in place of the bipyridine compound(DCbPyI), similar deposition results were obtained and theDCPhI was also subject to decomposition (decolorization) withbath aging. Although the decomposition mechanism has notbeen determined, it is obviously related to reaction withcadmium. It is apparently not an oxidation process and free
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iron is not detected in the decomposed bath. It is consideredlikely that the dissolved cadmium gradually replaces iron inthe compound (see Fig. 2) possibly forming cadmium bipyridineand releasing Fe(CN)2 . A pale yellow precipitate forms inthe aged bath but has not yet been analyzed for composition.Unless means can be found to retard, prevent or reverse thedecomposition process, practical use of the bath will belimited as the cost of the agents is considerable. It ispossible that the dicyanobis compounds would be usefuladditives in baths for plating other metals that may notbehave similarly to cadmium in enhancing decomposition. Forexample, the dicyanobis compounds may be useful additions foriron plating baths since decomposition would not be expectedin the presence of ferrous ions. Dicyanobis compounds ofmetals other than iron may be more stable and resistant todecomposition. For instance, if dicyanobis compound ofcadmium could be prepared, it should be stable in the cadmiumplating bath. A ruthenium analog is known but this compoundmay be prohibitive in cost even if it should prove effectiveand stable.
2.4 CATHODE CURRENT EFFICIENCY (CCE)
The final bath was tested for CCE based upon weight gainmeasurements using Faraday's law calculation. Deposits wereapplied in a rectangular cell with anode at one end andcathode at the other. The CCE was found to be 90 and 88% at5 and 25 mA/cm2 respectively.
2.5 HYDROGEN EMBRITTLEMENT
Preliminary tests were conducted to determine the tendencyfor producing hydrogen embrittlement in steel during platingfrom the newly developed bath compared to a bright cyanide-cadmium plating bath using the Barnacle Cell method [9]. Theamount of hydrogen pick-up was about 25% of that from thecyanide bath.
3. CONCLUSIONS
The Taguchi method of experimental design permits rapidevaluation of the effects of six bath variables since onlyeight plating baths were required. A bath formulation wasdeveloped which is considered potentially useful; however,aging of the baths results in serious decomposition of acritical and expensive agent, DCbPyI or DCPhI. Additionalinvestigation of the decomposition phenomenon and possiblemeans for avoidance is indicated.
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4. ADDITIONAL STUDI ES
4.1 BATH WITHOUT DICYANOBIS COMPOUNDS
Noncyanide cadmium bath of the following composition wasfound to provide semi-bright deposits over a wide cathodecurrent density range:
An approximation of Throwing Power (TP) was made by measuringthe thickness of cadmuim at several locations on the HullCell Ranel; 33% TP was found over the range of 10 to 40mA/cm4 cathode current density.
Extended use of the bath resulted in a gradual increase in pHindicative of higher anode current efficiency (ACE) thancathode current efficiency (CCE). When the pH increased toabove about 6.2, deposit adhesion problems were encountered;the pH must, therefore, be kept below about 6.0.
4.2 PROPRIETARY BATH (PAVCO INC)
The Ameribrite Neutra-Cad bath developed by Pavco Inc.,Cleveland, Ohio 44104, was investigated as an alternative tocyanide cadmium baths. Hull Cell tests were conducted anddeposits were quite bright but pitted. The efficiency ofdeposition (ACE) was determined using a Haring-Blum cell withthe results shown in the Table VI.
Table VI - Cathode Current Efficiency (CCE) ofProprietary Ameribrite Neutra-Cad Bath at Various
The ACE ranged from 102 to 112% with the higher values at thehigher current densities. Cadmium can therefore be expectedto build up in the bath with usage and would require use ofinert anodes such as graphite or lead.
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The TP was calculated to be 64% over the 10 to 40 mA/cm2 CCDrange. This is considered excellent and may be comparable tothe TP of typical cyanide - cadmium bath. Furtherinvestigation of this bath, in conjunction with the supplier,is believed warranted. The proprietary bath is currentlybeing used, apparently satisfactorily, at a large commercialplating facility.
5. RECOMMENDATIONS
5.1 Near-Neutral Cadmium
Conduct additional studies on the bath containing ATU, SMBT,MTA and Pluracol El000. The bath composition should beoptimized by utilizing Taguchi experimental design L1 6 (45)orthogonal array which entails preparation of 16 batks andfive variables, each at four levels. Baths should be aged toensure that all agents are stable. Determine ACE, CCE andTP.
5.2 Fluoborate Baths
Conduct tests with cadmium fluoborate baths at pH 4 to 5 withadditives to improve deposit appearance and throwing power.
5.3 Alkaline Bath
Conduct tests with alkaline - cadmium baths which entail useof complexing agents. Complexing agents of moderatestrengths, e.g., qluconates, quadrol, may be effectivewithout jeopardizing successful waste treatment. Alkalinebaths are normally excellent in TP.
5.4 Pulse Plating
Pulse plating (current interrupt or reversal) has beenreported to produce the following beneficial effects ondeposits from certain plating baths:
(1) smoother, denser, less porous deposits(2) increased deposition rate(3) improved current distribution (throwing power)(4) increased hardness and wear resistance(5) higher purity deposits(6) reduced hydrogen embrittlement
However, these effects have been noted for plating baths ofspecific composition using particular pulse wave forms. Mostprevious investigations have been carried out with pulsenterruption; relatively little with pulse reversal. Pulse
plating is an evolving technology and much more research anddevelopment work is required to effect advances in metal
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deposit characteristics to meet the needs of the military.
Pulse plating investigations are being considered in thefollowing areas of specific interest to NADEP platingfacilities:
(1) Cadmium plating of improved properties from non-cyanidebaths
(2) Improved throwing power of silver, copper, gold andnickel plating baths
(3) Alternatives to chromium plating:(a) Electroless nickel-phosphorus at increased rate &
hardness(b) Hard nickel(c) Hard cobalt(d) Tungsten-nickel alloy(4) Tungsten or molybdenum deposition (not achieved by
normal electrodeposition techniques)
The above areas will be investigated for feasibility andtechnology transfer to the NADEP's. Pulse plating equipmenthas been procured which is admirably suited for the proposedexperimental work because of the flexibility of the pulseformation and control and current capacity suitable forlaboratory or pilot lot studies.
5.5 Closed Loop Plating
Electroplating and other surface finishing operations at NavyDepots utilize hazardous/toxic chemicals such as cadmium,chromates and cyanides. The dragout in rinse waters must bewaste treated prior to discharge. However, restrictions onthe level of contamination of effluents entering waterways orsewage treatment plants are becoming ever more stringent and,limited landfill areas remain for disposal of sludges formedduring waste treatment operations.
Waste treatment of dragout chemicals in rinse waters isexpensive and represents a loss of nonrenewable resources;subsequent disposal of sludges represent substantial expensesand responsibility.
Technologies are currently available for economic recovery ofmetal values for reuse or sale and/or for recycling (closed-loop) of the chemicals back to the metal treatment tanks.
The following approach will be considered critically:
(1) Develop counterflow rinse requirements for cyanide-cadmium and chromium plating operations at selected NavyDepots.
(2) Construct appropriate counterflow rinse system at aselected Navy Depot.
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(3) Determine minimum rinse water flow rate to achievedesired degree of parts cleanliness and theconcentration of chemicals in effluent.
(4) Determine practicability of recovery of cadmium byelectrodeposition along with anodic cyanidedecomposition.
(5) Determine degree of evaporation of treatment tank(cadmium and chromium plating baths) to enable return ofall counterflow rinse waters; i.e., closed-loop plating.
(6) Procure appropriate atmospheric evaporators to effectclosed-loop plating (consideration will also be given tovacuum distillation or advanced reverse-osmosis forconcentrating chemicals in the rinse water).
(7) Conduct Navy Depot evaluation of process.(8) Evaluate process for cost savings and intangible
benefits.
6. REEERE{CE
1. D. E. Muelberger, Plating and Surface Finishing. 70, 25(1983).
2. N. Zaki, Metal Finishing. 187, 57 (1989).3. J. M. Biehl, Proceedinas of Workshop on Alternatives to
Cadmium ElectroDlating in Metal Finishing. USEPA, Officeof Toxic Substances, Washington, DC, Oct.1977, pp.259-287.
4. E. A. Davis, ibid, pp. 409-422.5. H. G. Bates, "Hazardous Waste Minimization Research
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