ALTERNATIVE FOR TANK CHROMIUM USING (BRUSH) PLATING

JohnLangan SIFCO Selective Plating

Division of SIFCO Industries 5708 Schaaf Road

Cleveland, Ohio 441 31

ABSTRACT

The deposit properties of hard chromium provide several advantages in many industrial applications, however, there is one significant disadvantage associated with hard chromium plating. Hard chromium plating is an environmental concern of such magnitude that the need to seek out and consider alternative deposits is justified.

It is a tall order to fill for an alternative deposit to provide all the performance characteristics of hard chromium. The intention of this writer is to evaluate properties of some brush plated deposits and propose them for consideration as possible alternatives to hard chromium plating.

To provide support as to why the proposed brush plated deposits should be considered as alternatives to hard chromium, advantages and disadvantages of the hard chromium bath plating process and the brush plating process will be discussed.

BACKGROUND - HARD CHROMIUM

Hard chromium for many years has enjoyed a reputation as being an electroplated deposit having many special properties serving several purposes. Deposit properties such as extremely hard deposits, low coefficient of friction, excellent wear resistance, corrosion resistance and oil retaining capabilities all have given hard chromium plating recognition in a multitude of applications. Industrial applications have steadily increased for several decades.

The commercial process of chromium plating was largely through the efforts of Fink and Eldrige back in 1923 and 1924’. The historical value in itself gives the user of hard chromium plating confidence for the chromium plating process is well known and accepted. Hard chromium plating is well tried and true and has proven itself for many years.

A common purpose of hard chromium plating is to rebuild or salvage worn parts. Examples such as rolls, roll journals, moulding dies, internal combustion engine cylinders, crankshaft journals and heavy duty shafts for presses or turbines are typical areas where hard chromium is deposited and its unique properties are utilized.

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AESF Annual Technical Conference SUWF~N~ TBZE

Junes 2 2 2 C - Z 5 # 1S-S Atlanta, Georgia

The American Electroplaters and Surface Finishers Society, lnc. (AESF) is an international, individual- membership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as govemment agencies and the general public. AESF dissemi- nates technical and practical information through its monthly joumal, Plating and Surface finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry.

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WHY HARD CHROME?

Hard chromium deposit properties are considered to provide several advantages. For example, chromium's hardness is often considered a deposit property giving the advantage of wear resistance. However, the hardest deposits do not necessarily give the greatest wear resistance. Some investigators* reported that wear resistance was independent of hardness. For example, with 0.0008 in. or thicker coatings, no difference was observed in deposits with a 750 to 950 Brinell hardness. In typical applications, chromium deposits' low coefficient of friction is largely responsible for deposit wear resistance. The low coefficient of friction of chromium deposits is an important advantage utilized on applications such as piston rings, internal-combustion engine cylinders, crankshafts and similar applications.

Chromium deposit modifications, either by mechanical means such as grit blasting the basis metal, chromium deposit and grind to size or by chemical means such as etching solution treatments, create a porous or pitted deposit. This modified chromium deposit, known as porous chromium plate, exhibits the advantage of oil retaining properties desir d in applications such as internal- combustion engine cylinders and piston rings P .

Another advantage of hard chromium plating is its ability to retain hardness after elevated temperatures. This property is particularly useful in aircraft engine applications.

DISADVANTAGES

The use of chromium plating is an environmental concern of great magnitude. Alternatives to chromium plating are strongly desired especially when considering it is included in the ''four C's'' hit list (chromium, cadmium, cyanide, and chlorinated solvents).

Environmental and workplace health hazards are disadvantages associated with hard chromium plating. The hexavalent compounds of the chromium metal are extremely toxic and hazardous.

Typical of hard chromium plating is large amounts of hydrogen developing at the cathode and oxygen at the anode. As these gases escape from the bath, a mist is formed carrying with it chromic acid. Chromic acid, if permitted to escape into the workplace, is not only damaging to the surroundings but also constitutes a health hazard. To assure the safety of personnel in the workplace, as well as prevent heavy fines by OSHA and or the EPA, it is mandatory to have adequate ventilation and scrubber systems.

Adding to environmental concerns is the method of wax masking often associated with hard chromium plating. ,4f?er chrome plating is complete, the wax Is stripped by methods such as hot wax dipping or using a steam autoclave. These methods do not remove the wax entirely, therefore, residual wax left on the part is then stripped using potentially hazardous solvents such as perchlorethylene or 1,1,1 trichloroethane. Solvents used in cleaning as well as the removed wax are considered hazardous waste and add to disposal problems3.

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OPERATIONAL DISADVANTAGES

Operational difficulties or complications typical of the hard chromium bath plating process add to its disadvantages. Cathode efficiency is low. Conventional hard chromium baths have a cathode efficiency of 7-15%. Higher cathode efficiencies ranging from 20-25% can be achieved with some proprietary hard chromium baths.

Along with a low cathode efficiency, hard chromium plating has a low deposition rate. To produce a good quality chromium deposit, a deposition rate as low as 0.001 in. per hour is typical. Long plating times from several hours up to days at a time are often a necessary evil creating lengthy turn-around times and production delays.

Hard chromium bath plating's poor throwing power will add to production delays. Poor throwing power can lead to insufficient deposit thickness when plating tight access areas such as into shoulders or blind holes. Parts are often plated with excessive deposit thickness to compensate for this and assure adequate material has been deposited into tight areas. This over plate compensation leads to longer plating times and adds to production delays.

Because the hard chromium plating bath exhibits poor throwing power, extra consideration is needed for the design and use of plating racks and fixtures. Critical in thick deposit applications is :he use of conforming anodes to increase local current density in recessed areas .

The shape, as well as the accessibility of the area to be plated, can create complications when designing auxiliary or conforming anodes. The positioning of the anodes in relation to the cathode may be restricted due to the part's geometry. Tight recessed areas along with tank chromium's poor throwing power is a combination that can lead to uneven plating buildups and possibly insufficient deposit thickness required to restore dimension.

Often with chromium plating, the entire part is immersed into the bath. For this reason, the entire part requires masking except for the area being plated. Masking the part by the typical wax process is time consuming in both application as well as removal time. With the part being immersed into the bath for what can be days at a time, there is justified concern for the part's welfare, especially when dealing with extremely valuable parts often associated with the aircraft industry. A leak in the masking could be catastrophic.

plating processes is the disassembly requirements. Before a part can be h"rsed into the bath, it typically requires full disassembly of all other components to assure their safety. Examples would be sensitive aircraft electrical components such as generators or motors. Disassembly is yet another time consuming requirement adding to production delays.

BRUSH PLATING

Another disadvantage of not only chrmiun l~th~pplating but mst bath

The process of brush plating should be recognized for its historical background much like the process of hard chromium bath plating. Through the

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innovations of Mr. George lxci back in 1938 in Paris, France, the brush plating process was developed. The brush plating process was introduced into the United States in the late 1950's. Since its introduction into the United States, brush plating has continuously been improved. Improvements in equipment, techniques, variety of applications, preparatory and ' plating solutions and deposit quality has led to increasing acceptance in commercial and government specifications5.

Brush plating deposits can be used to serve some of the same purposes as hard chromium plating. Brush plating deposits can provide corrosion protection, improve wear resistance, prevent galling and salvage worn or mismachined parts. Unfortunately, there is no one single brush plating deposit that has all the properties of hard chromium. However, there are applications where hard chromium is specified but the application may not necessarily need all that hard chromium has to offer.

Consider for example a bearing journal on a shaft that has galling damage. The standard repair procedure specifies to use hard chromium to restore the bearing journal surface. In this general example, hard chromium may not necessarily be the best choice of deposit. Hard chromium, having a low static coefficient of friction, may not serve as well as a deposit such as nickel. The nickel deposit, having a higher static coefficient of friction may actually serve better in keeping the bearing's inner race stationary on the bearing journal.

Applications specifying hard chromium plating should be evaluated on a case by case basis and a determination made as to whether hard chromium is being specified because of tradition, "that's how it has always been plated", or if hard chromium is absolutely needed for specific deposit properties that no other coating can provide. Certain brush plating deposits have similar deposit properties of hard chromium and if properly matched with the right type of application, can serve as viable alternatives.

There are certain benefits that can encourage the use of brush plating when taking into consideration some of the advantages the process of brush plating has to offer over hard chromium bath plating.

BRUSH PLATING ADVANTAGES

Masking requirements are minimal and there is a lower risk of electrochemical attack since parts being brush plated do not require immersion into preparatory and plating solutions. Parts can be plated without disassembly saving many man hours. Typically only the adjacent areas to the area being plated require masking which is done using aluminum and vinyl tapes. After masking tapes are adequately rinsed, they can then be discarded without the hazardous waste concerns encountered with wax masking.

Brush plating has an advantage over bath plating with regards to plating speed. Higher plating currents are used when brush plating as compared with bath plating. Brush nickel, for example, is plated at an average current density of 7 amplsq in. Fast rates of deposition, (0.035 in./hr. as an average for brush nickel) and the capability to plate at high current densities are largely due to excellent plating solution agitation at the cathode that is provided by the "brushing" action of the anode.

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The direct anode-to-cathode contact, achieved when brush plating can improve deposit distribution and reduce uneven plating buildups. The brush plating anode can direct the plating into tight access areas such as blind holes or inside corners and reduce the tapering effect often encountered when hard chromium bath plating these same areas. Both plating as well as machining times are reduced with less "overplate" needed to compensate for deposit tapering.

The volume of hazardous waste generated from brush plating is considerably lower when compared with waste volumes generated from hard chromium plating. This is a significant advantage when you consider today's high cost for disposal and treatment of hazardous waste. Additional advantages of the brush plating process are:

- Portable: The brush plating process can be brought to the part vs. bringing the part to the tank.

- Simple: Preparatory and plating solutions do not require adjustments in chemistry as would be the case with tank plating.

- Accurate: Capable of plating to the exact desired thickness. - Versatile: Allows plating parts that are too large for existing tanks.

DISADVANTAGES

Brush plating is not suitable for plating complex shapes. For example, the entire outer cylinder of a main landing gear requiring cadmium plating would be better suited for bath plating due to the part's complexity. Another disadvantage is that brush plating can be labor intensive. Typically the plater only processes one part at a time.

DEPOSIT QUALITIES

As mentioned earlier, there is not one individual brush plating deposit offering all the deposit properties of hard chromium. There are, however, certain brush plated deposit that do have similar properties and are useful in salvage/repair type applications. The following will provide some qualitative and quantitative data on certain brush plating deposits used in salvage/repair type applications.

ADHESION - Using qualitative adhesion tests listed in ASTM B-57I6, brush plating deposits can be evaluated and compared to other coatings.

Figures 1 and 2, compressive and tensile bend tests respectively, demonstrate the excellent adhesion and cohesion of hard (575 DPH) brush sulfamate nickel deposits. These destructive bend tests also show that this deposit exhibits fair ductility. For comparison, figures 3 and 4 show the results of the same destructive bend test run on hard (900 DPH) chromium bath plated deposits. Note in figure 3 the hard chromium under compressive bending failed both adhesively and cohesively.

Blunt chisel tests of hard brush sulfamate nickel (figure 5) and hard chromium, bath plated, (figure 6) show the brush sulfamate nickel deposit exhibits better cohesive properties than the hard chrome after sharp impact.

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FIGURE 1. Compressive bend test, hard (575 DPH) brush sulfamate nickel, 0.0044 in. thick, on low carbon steel.

FIGURE 2. Tensile bend test, hard (575 DPH) brush sulfamate nickel, 0.0043 in. thick, on low carbon steel.

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I)

FIGURE 3. Compressive bend test, hard (900 DPH) chromium, 0.0087 in. thick, on low carbon steel.

FIGURE 4. Tensile bend test, hard (900 DPH) chromium, 0.0087 in. thick, on low carbon steel.

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FIGURE 5. Blunt chisel test, hard (575 DPH) brush sulfamate nickel, 0.0037 in. thick, on low carbon steel.

FIGURE 6. Blunt chisel test, hard (900 DPH) chromium, 0.010 in. thick, o n low carbon steel.

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TENSILE BOND - Two different brush nickel deposits were deposited onto SAE 4130 steel and tested in accordance with ASTM C 633-7g7 to quantitatively test their tensile bond strength.

The first nickel tested was a hard (585 DPH) high speed neutral nickel. The test results report that the samples all failed in the cement joint at an average of 11,280 psi8. The results indicate that the cohesive strength and adhesion of the hard nickel on 41 30 steel is at least 11,280 psi. It should be noted that this deposit is ranked as having only fair adhesion per brush plating standards, yet this deposit exhibits superior adhesion to flame spray coatings and is at least equivalent to thermal plasma spray coatings.

The second nickel tested was soft (250 DPH) brush sulfamate nickel. Again, the samples failed in the cement joint, this time at 10,090 psi. The test report concludes that the adhesion of the nickel to steel, the cohesion of the nickel and the adhesion of a second layer of nickel deposited to a first, therefore, exceeds 10,090 psig.

SHEAR LOAD - To quantitatively measure the bond stqsngth between layers of bryFh sulfamate nickel, the ASTM Ring Shear Test Method was used. For this study the following deposit combinations were tested:

A - Soft sulfamate nickel on soft sulfamate nickel.

B - Hard sulfamate nickel on soft sulfamate nickel.

C - Hard sulfamate nickel on hard sulfamate nickel.

The results of the test were as follows:

TEST psi STRENGTH* LOCATION OF FAILURE

A 48,700 In deposit

B 50,100 In deposit

C 49,400 Between layers

* Bond strength values are from shear load tests and should not be directly compared to bond strength values derived from tensile tests.

TABER WEAR TEST - Abrasive wear tests were run on hard chromium bath plating, several brush plating deposits and a few base materials for the purpose of comparison’*. The conditions of the test were as follows:

Three hundred wear cycles using CS-17 abrasion wheels and 1000 gram load were conducted. The loss in weight was noted and the abrasion wheels were then redressed.

The results show (see Table 1) as other testing has shown, that a high hardness does not necessarily correlate with good abrasive wear resistance.

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Bath Hard Chrome (900 DPH)

Brush Chrome (584 DPH)

Brush Cobalt Tungsten (540 DPH)

SAE 4340 @ 50 Rc (51 3 DPH)

Brush Neutral Nickel (585 DPH)

304ss (1 54 DPH)

SAE 4340 @34Rc (332 DPH)

Brush Nickel (Acid) . (1 79 DPH)

Brush Cobalt (441 DPH)

Low Carbon Steel (75 DPH)

TABLE 1

TABER WEAR TESTS

WEIGHT LOSS (IN GRAMS)

300 600 900 1200 1500 Total Weight cycles cvcles cvcles cvcles cvcles Loss in Grams

.0011 .oooo .oooo .0007 .0006 .0024

.0012 .0006 .0004 .0002 .0002 .0026

,0031

.0050

.0049

.0039

.0046

.0048

.0107

.0095

.0017

.0030

.0031

.0030

.0035

.om2

.0048

.0078

.0023

.0034

.0034

.0038

.0042

.0048

,0051

.0082

.0022

.00,18

.0023

.0020

.0038

-0047

.0059

.0076

.0015

,001 6

.0022

.0038

.0037

.0044

.0066

.0075

.0108

.0148

.0159

.0165

.0198

.0229

.0331

.0406

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HEAT RESISTANCE - In certain applications, a desirable property of hard chromium is the ability to maintain hardness after elevated temperatures. Cobait-tungsten brush plated alloy deposits share with hard chromium this same property. Figure 7 shows the effect of temperature on deposit hardness after two hour heat treatments. It is interesting to note that the cobalt tungsten maintained a higher hardness after 12OOOF than the hard chromium (540 DPH vs. 360 DPH).

900 -

800 --

700 --

600 --

o m

H 400 --

-- P

PLATED)

COBALT-TUNGSTEN (BRUSH PLATED)

t 0 4 I

0 200 400 600 800 loo0 1200 1400 1M)o

DEGREES FAHRENHEIT

FIGURE 7. Deposit Hardness After Two Hour Bake

Other tests13 of cobalt-tungsten brush plating were conducted on deposits of 0.004 in. thick plated onto low carbon steel. The samples were heated for two hours at 900°F, 1200OF and 1500OF in an ordinary oxidizing atmosphere. The following results were reported:

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A.

B.

C.

Visual Examination and Bend Test Results

As Plated After 900OF After 12OOOF After 150OOF

Metalloqraphic Results

As Plated After 900OF After 12OOOF

After 15OOOF

Microhardness Tests

100 qram load D.P.H.

As Plated After 9OOOF After 12OOOF After 150OOF

Adhesion good. Adhesion good. Light oxide. Adhesion good. Scale, loose 0.001 in. thick. Adhesion good. Scale, loose 0.002 in. thick.

Scattered stress-cracks. Scale at surface and around cracks. Scale 0.004 in. thick on surface. Scale around cracks. Scale and internal oxidation 0.0005 in. deep from surface and cracks.

Average of 3 readinas

493 503 493 435

Current state of the art of cobalt-tungsten brush plating does not provide the capability to produce appreciable deposit thickness. This limitation minimizes cobalt-tungsten's usefulness with salvage/repair type applications. However, the property of sustained hardness, after being subjected to elevated temperatures, is a deposit quality justifying further research and development of cobalt-tungsten alloy plating. .

Brush sulfamate nickel plating, often used for salvage applications, was also tested14 to determine the effect heat has on deposit hardness. There are three hardness ranges available from brush sulfamate nickel plating solutions and all three were tested (see figure 8). The test results indicate the hard brush sulfamate nickel deposits (400 and 575 DPH) are best if used in operating service temperatures not exceeding 5OOOF.

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- SULFAMATE NICKEL (575) - SULFAMATE NICKEL (400)

.-+-- SULFAMATE NICKEL (250)

P

I I

7 i

0 100 200 300 400 500 600 700 800 900 loo0 DEGREES FAHRENHEIT

FIGURE 8. Brush Sulfamate Nickel Deposit Hardness After Four Hour Heat Treatment

S U M MARY

The brush plating process offers nineteen pure metal plating solutions and several alloy plating solutions. The large selection of plating solutions available provide a variety of deposit properties that can meet many application requirements. Deposit quality, cohesion, and adhesion to the base material are equivalent or superior to good bath plating practice.

Hard chromium plating cannot be entirely replaced using brush plating deposits, however, hard chromium usage can be reduced by carefully evaluating the application and determining if hard chromium is absolutely essential. If it is determined that an alternative plating deposit (for example sulfamate nickel) can meet the requirements of the application and the brush plating process is used to apply the alternative deposit, then significant advantages will be gained. The most significant advantage would be an environmental advantage through the minimiza- tion of hard chromium use.

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REFERENCES:

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

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G. Dubpernell in F.A. Lowenheim, Ed., Modern Electroplating, 3rd Ed., Wiley, New York, 1974, pp. 87-1 38.

W.H., Safranek., The Properties of Electrodeposited Metals and Alloys, American Elsevier, New York, 1974, pp. 33-61.

John Langan, Mary Caporaso., "Brush Plating Applications Utilizing AeroNikl@ Sulfamate Nickel on Aircraft Components," SAE Technical Paper 91 0938, March 1991.

Greg Piner, Gary Whitfield., "Production Advantages of Hard Chromium Plating Using Close Anode to Cathode Spacing," SAE Technical Paper 870739, February 1987.

SlFCO Selective Plating, DALIC Process Instruction Manual, 5th Ed., 1990.

ASTM Standard Test Methods For Adhesion of Metalic Coatings, 6571 -84.

ASTM Standard Test Method For Adhesion or Cohesive Strength of Flame Sprayed Coatings, C633-79.

SIFCO Selective Plating, Technical Service Bulletin No. 72, "Tensile Bond Test on Nickel Deposits, Code 2085."

SlFCO Selective&lating, Technical Service Bulletin No. 101 , "Tensile Bond Test on AeroNikl 250 Code 7280 Deposits."

ASTM Standard Test Methods for Shear Testing of Porous Metal Coatings,

Michael Moskowitz., "Use of Sulfamate Nickel Brush Plating for Heavy Build- Up," AESF Technical Paper, June 1991.

SlFCO Selective Plating Technical Service Bulletin No. 55, "Evaluating the Abrasive Wear Resistance of DALIC Deposits.''

SIFCO Selective Plating Technical Service Bulletin No. 56, "Effect of High Temperatures on DALIC Deposits."

Gary W. Smith., "Advanced Selective Plating with AeroNikl@ Solutions," SAE Technical Paper 890935, March 1989.

F1044-87.

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