Methods to Improve the Corrosion Performance of Microporous Nickel Deposits

Methods to Improve The Corrosion Performance Of Microporous Nickel Deposits

By Robert A. Tremmel

During the last several years, nickel, which is essentially sulfur- microporous nickel-chromium free, plus a thinner layer of bright coatings have been critically nickel. Duplex nickel provides scrutinized by the automotive corrosion protection that is far industry. Today these coatings must superior to single layer systems. In be free of all surface defects, even these duplex systems, when corrosion after long-term exposure to acceler- occurs through a pore in the chro- ated tests and in real-life service. mium plate, the bright nickel is While proper control of all the rapidly penetrated until the semi- multi-layer nickel deposits is bright nickel is reached. Because the important, blemish-free surfaces electrochemical potential of the semi- Fig. 1—Corrosion mechanism of decorative nickel- can only be obtained when the chromium deposits. bright deposit is greater than that of microdiscontinuity and the activity the bright nickel deposit, thereby of the post-nickel strike are prop- making it more noble, the bright erly achieved and maintained. This also be free of surface defects1. nickel layer will corrode preferen- edited version of a paper presented Simply meeting porosity specifica- tially to the semi-bright nickel layer. at SUR/FIN® ’96—Cleveland shows tions is not enough. The size of the As the pit widens, however, some that research indicates that the pores is important, as well as the attack will eventually occur in the most important factor is the electrochemical potential of the semi-bright nickel layer and ulti- electrochemical potential of the microporous nickel strike. This latter mately penetrate the basis metal. microporous nickel strike deposit. factor could be the most important. Figure 2 illustrates the corrosion cell.

ecorative nickel-chromium has Corrosion Mechanisms been used as a protective Bright Nickel-Chromium D coating on a variety of The corrosion of decorative nickel- substrates for more than 70 years. chromium coatings is electrochemical During this time, significant improve- in nature. A solution of salt, and ments have been in the corrosion especially acidic salt solutions Fig. 2—Corrosion mechanism of duplex nickel resistance of these platings. This is encountered in industrial winter deposits. especially true for multi-layer atmospheres, sets up a galvanic cell coatings developed for highly whereby the chromium deposit corrosive environments. The corro- becomes cathodic while the underly- High Sulfur Nickel sion mechanisms of multi-layer ing nickel layer, exposed in the cracks Attack on the basis metal can be coatings are very well understood. and pores of the chromium layer, further delayed by either increasing The literature, however, has often becomes anodic. The large cathodic the sulfur content of the bright nickel been inconsistent with respect to the areas of the surface of the chromium layer (making it more active) or by recommendation of specific operating and the very small anodic areas of the incorporating the use of a very high parameters and the use of additives to exposed nickel are precisely the sulfur nickel strike between the semi- obtain the desired conditions required conditions that favor rapid corrosion bright and bright nickel layers. As the for maximum corrosion performance. pitting of the nickel deposit. Very potential difference between the In the past, the main requirement of little nickel is corroded but, because bright and semi-bright layers is nickel-chromium coatings was the the electrochemical attack is concen- increased, the rate of penetration to prevention of base metal corrosion. trated at a small point, there is rapid the basis metal will be decreased. Surface blemishes and a certain penetration to the basis metal. Figure Figure 3 illustrates the corrosion amount of surface dulling was 1 illustrates this corrosion cell. mechanism of a high sulfur nickel allowed. These surface defects usually strike. occurred because of the deterioration Duplex Nickel-Chromium Note that the use of a thin high sulfur of the microdiscontinuous deposit(s). Duplex nickel, currently required for nickel strike [0.050 µm(0.000020 in.)] Today, multi-layer nickel-chromium all exterior coatings by the automotive can double corrosion protection of the coatings must provide very long-term industry, was developed in the 1950s. basis metal. This is especially true in base metal corrosion resistance and It consists of a layer of semi-bright

24 PLATING & SURFACE FINISHING 3. A microporous nickel strike that Fig. 5—Effect of electrochemical induces microporosity in the potential on subsequent chromium layer. This number and size is achieved by either co-deposit- of corrosion ing or trapping inert particles in sites. The potential the nickel strike deposit. Gener- Fig. 3—Corrosion mechanism of high sulfur between the nickel deposits. ally, microporosity from this microdiscontinuous deposit can be as high as 155,000 nickel and the 2 2 bright nickel in areas where the nickel thickness is pores/cm (1,000,000/in )* the top photo is low. 4. Impinging the chromium deposit a minus 20 mv. with sand or other fine, hard The difference Microdiscontinuous Chromium particles to produce micro porosity. in the bottom photo is a plus The advantages of microdiscon- This method can produce micro- 30 mv. tinuous chromium with respect to the porosity as high as 100,000 pores/ overall corrosion protection of single cm2 (650,000 pores/in2). and multi-layer nickel coatings was 2 the nickel strike is less electronega- discovered in the late 1950s. It was Of the four methods, only the last tive (more noble) than the bright initially believed that the use of crack- two are used extensively in the plating nickel underlayer, outstanding free chromium would provide a industry. Microcracked chromium, corrosion protection can be achieved completely corrosion resistant surface whether produced in the chromium when used over standard duplex as there would be no exposed nickel deposit itself or by a stressed nickel nickel coatings. Within certain to set up corrosion cells. While crack- strike, has some serious limitations. prescribed limits, increasing the free chromium can be plated, internal These include low crack density in the micro-porosity will decrease the size stresses develop in relatively short low current density areas and a of the corrosion sites and improve the periods of time causing cracks to tendency to flake off in the extreme overall corrosion protection. Ideally, form. This results in corrosion sites high current density areas. for optimum cosmetic and base metal developing in the same manner as on protection, the microporosity should normal chromium. Microporous Nickel Strike be between 16,000 and 48,000 pores/ Microdiscontinuous chromium Microporous chromium, which is cm2(100,000 and 300,000/in2). When provides unique corrosion mecha- obtained by impingement, provides the porosity is in this range and the nisms that significantly improve excellent porosity but its use is desired potential of the nickel strike is overall corrosion protection. generally limited to simple shapes. It met, plated parts can withstand up to Microdiscontinuity in the chromium also requires a relatively high capital 10 cycles of CASS* without any base layer decreases the cathode area (the investment. The most widely used metal or cosmetic defects. Figure 5 chromium) and increases the anode method is the incorporation of a illustrates the effect of electrochemi- area (the nickel). As a result, the microporous nickel strike prior to cal potential on the size of the amount of current supplied to the chromium plating. This method, corrosion sites. nickel at a given corrosion site is however, has some limitations. substantially less, and penetration Because the particles are inert, for Additives through the bright nickel layer is example, they do not readily migrate Historically, the major problems with reduced. Figure 4 illustrates the to the cathode surface, especially to microporous nickel strikes have been corrosion mechanism of microdis- the under-shelf areas that are usually the inconsistent maintenance of the continuous chromium. the most important surfaces on the desired porosity and achieving the part. Furthermore, if the particles are desired electrochemical potential. too large, the subsequent corrosion Recently, however, improvements sites will be large and highly have been made to overcome these undesirable from the standpoint of Fig. 4—Corrosion mechanism of microdiscontinuous limitations. Some of these improve- chromium. appearance. ments have been made through the The electrochemical potential of the revision of process parameters and the Ways To Achieve microporous nickel strike layer also rest by new and improved addition Microdiscontinuity plays a significant role in relation to agents. Microdiscontinuity can be produced the size of the corrosion sites. If the Very fine powders have been by several techniques. These include: nickel strike layer is more active than developed that co-deposit much more the bright nickel layer below it, it will readily than previous powders. Liquid 1. Highly stressed chromium deposits corrode preferentially to that layer suspensions are now being supplied to that provide up to 1500 microcracks and cause significantly larger corro- facilitate additions. While the older per linear inch. sion sites than if less active than the powders required peroxide additions 2. A microcracked nickel strike that bright nickel. and electrolysis to activate them, the induces microcracking in the newer powders are activated immedi- subsequent chromium layer. This is Microporosity & ately and require no break-in period. achieved by highly stressing a thin Electrochemical Potential nickel layer that is plated over the Test results have shown that if the * Copper Accelerated Salt Spray. The benchmark test currently used to evaluate nickel-chromium-plated bright nickel prior to plating microporosity exceeds 10,000 pores/ automotive trim for exterior exposure. See ASTM 2 2 chromium. cm (64,000/in ) and if the potential of Standard B 368-85 for details.

October 1996 25 The co-deposition rate of these very Chrysler Specification PS 8908 microdiscontinuous nickel layer must fine powders is much higher than with states that the corrosion sites, as be substantially more noble than the previous solids. Consequently, much examined after CASS, must be no bright nickel underlayer. less material is required in the bath to larger than 63µm (0.0026-in.) with an Joint tests made by Ebara-Udylite achieve desired porosity. In fact, as average size of about 31µm (0.0013-in.). and Toyota in Japan indicate that little as 0.5 gal/L is often sufficient. there are relative potentials between Older formulations required solid the microporous strike layer and the concentrations as high as 7.5 gal/L. bright nickel that will guarantee Consequently, the new powders are virtually blemish-free surfaces, even easily removed by filtration, which after 200 hours CASS.9 Using an facilitates bath purification proce- electrolytic film thickness meter and a dures and reduces treatment time and multi-layer Ni corrosion resistance cost. meter, absolute potentials were Other special additives enhance the measured on various bright nickel co-deposition of the inert powder. It is and microporous nickel strike believed that these “activators” cause deposits. Results were compared to electro-osmotic effects in the cathode their corrosion performance in CASS. film that make the particles cling to Fig. 6 indicates that the strike deposits the cathode surface and facilitate co- must be 20 to 40 mv more noble than deposition.5 They work particularly the bright nickel underlayer to achieve well with the new, improved powders the desired corrosion protection and to enhance under-shelf (“A Surface”) Fig. 6- Shaded area indicates desired potential for appearance.10 optimum cosmetic and base metal corrosion porosity. Small additions can double protection. or even triple microporosity. Special Addition Agents In the past, platers tried to minimize Agitation Electrochemical Potential potential differences between the two Agitation plays a major role with The size of the corrosion pit is layers by frequent pump-outs and respect to particle co-deposition. Air mainly determined by the electro- treatments—sometimes as often as agitation is important to keep the chemical potential of the microporous weekly. The solids that can be particles suspended, however, it tends nickel deposit, especially with respect deactivated by the adsorption of to blow the particles away from the to its relationship to the bright nickel impurities were usually changed after surface of the cathode, which reduces deposit. For example, when corro- pump-outs. Often the solids were co-deposition. Pulsing the air has sion takes place in the micropores, filtered off and the strike bath mixed been recommended by Manquen and if the strike is more active than the with the bright nickel in order to has provided some improvement.6 In bright nickel underlayer, the strike equalize the potential. While these this case, the air is turned on and off deposit will be anodic to the methods helped, they would rarely, if for specific periods of time. A typical underlying nickel deposit and ever, produce a microporous nickel cycle might be 15 sec on, 30 sec off. corrode preferentially to said nickel strike deposit that was more noble Ideally, the use of air agitation should layer. This will substantially than the bright nickel coating. be minimal — only enough to keep increase the size of the corrosion Test results indicate that the the solids uniformly suspended. The sites, which can cause highly visible composition and concentrations of the majority of the plating time should be and unsightly blemishes and/or organic addition agents used in the without agitation. stains to occur on the surface of the strike bath can have a significant The use of the proper additives, electrocoating. Conversely, if the effect on the electrochemical potential coupled with properly managed strike layer is less active than the of the subsequent deposit.11 Usually, agitation, can provide pore densities bright nickel underlayer, said the activity of a nickel deposit is up to 50,000/cm2(312,500/in2) on underlayer will corrode preferentially, determined by the amount of sulfur significant surfaces and up to leaving the corrosion site at the included in the deposit during 160,000/cm2 (1,000,000/in2) on top surface almost invisible. As a result, electrodeposition — the higher the shelf areas. the plated surface will be virtually sulfur the more active the deposit. free of blemishes, even after extensive This means that the deposit potential Pore Size exposure to severely corrosive is more electronegative. High porosity, within limits, is environments. Class I brighteners such as saccha- important, as it not only reduces Many automotive industry stan- rin, benzene sulfonic acid, benzene corrosion penetration, but also dards specify that in order to mini- sulfonamide and benzene or toluene reduces the initial size of the pores. In mize pore size after CASS, the sulfinic acids provide sulfur that is co- general, the smaller the initial size of electrochemical potential (measured deposited with the nickel. The amount the pore, the smaller the eventual by a STEP tester) of the microdis- of sulfur included in the deposit varies corrosion site. This is important continous nickel layer should be depending on the additive(s) used. Of because the smaller the corrosion within plus or minus 20 millivolts of the ones listed, saccharin, which is sites, the clearer the deposit will be, the bright nickel.7,8 A difference of widely used in bright nickel plating, even after long-term exposure to zero was considered to be ideal. We provides the least amount of sulfur. severely corrosive environments. find, however, that in order to Benzene sulfinate provides the most. ensure blemish-free surfaces, the

26 PLATING & SURFACE FINISHING Class II Brighteners, when used in by minimizing the sulfur inclusion in strike can provide rust and blemish- combination with Class I Brighteners the strike, adding the supplier’s free surfaces even after exposure to to produce brightness and leveling, recommended additives to facilitate 200 hours CASS. P&SF also help to increase the sulfur content carbon co-deposition and low pH, of the bright nickel deposit. Because carbon treating the strike as required References they usually contain no active sulfur, (usually every four to six weeks). The 1. R. J. Clauss, R. Tremmel, R. E. they do this by causing the Class I desired porosity in the nickel strike is Fischer and E. Hoover, Products Brightener to plate more sulfur into now more easily obtained through the Finishing, 60 (March 1983) the electrodeposit.Organic degrada- use of finer, more active powders, 2. G.A. Dibari, Metal Finishing, 75, 3 tion products that tend to build up in a pulsed air agitation and the addition (June 1977) bath over a period of time can also of special activating agents. Assuming 3. Standard Specification B456-85, increase the activity of the deposit. that the STEP between the semi- Appendix 4, ASTM, Philadelphia, These are generally controlled with bright and the bright nickel deposits is PA. continuous carbon purification or in the desired range, and thickness 4. E. P. Harbulak and E. W. batch treatments. specifications are met, multi-layer Romanowski, Plating & Surface Because the bright nickel bath is deposits with a noble microporous Finishing, 76, 58 (March 1989) usually dragged directly into the microporous nickel strike, it is often difficult to maintain the desirable potential difference. Usually, the microporous strike deposit is more active than the bright nickel deposit because the bath cannot be continu- ously purified. This, in turn, allows degradents to build up. This creates the need to batch-carbon treat on a regular basis. Certain additives can be used in decorative nickel plating baths to control their electrochemical potential. An example of the effect of carbon and sulfur inclusion on electrochemical potential can be seen in Table 1. The data indicate that the inclusion of sulfur into the electrodeposit makes the deposit more active, electrochemically. Con- versely, the inclusion of carbon into the electrodeposit makes it more noble. In practice, before the addition of desired additives, the potential of a microdiscontinuous nickel strike is generally more electronegative than the bright nickel deposit. After the desired materials are added, the deposit becomes much less electronegative. These results demonstrate that by carefully control- ling the types and the amounts of certain additives in both the bright nickel and the microporous strike baths, the desired potentials can be achieved.

Multi-layer Nickel Coatings Can Improve Performance In conclusion, when using multi-layer nickel coatings, outstanding cosmetic and base metal corrosion protection can be achieved when the electrochemical potential of the microporous nickel strike is 20 to 40 mv more noble than the underlying bright nickel deposit. This can be achieved Free Details: Circle 113 on postpaid reader service card.

October 1996 27 Hatanaka, H. Wada, “Improvement Effect of Sulfur and Carbon on Electrochemical Potential of the Corrosion Resistance of Of Nickel Deposits Decorative Chrome Plating.” Presented at the Surface Finishing Test No. % Sulfur % Carbon Potential Society of Japan, 87th Conference, (mV) p. 253 (March 1993) 10. Ibid. 1 0.001 0.017 0 2 0.004 0.007 -60 11. A. Fukada, N. Ozawa, T. 3 0.032 0.003 -180 Fukushime, M. Kamitani, M. 4 0.033 0.031 -110 Ohgane, “Effects of Sulfur and 5 0.036 0.220 0 Carbon in Nickel Plating Deposits 6 0.053 0.067 -90 7 0.088 0.240 -40 on Corrosion Potentials.” Pre- 8 0.172 0.005 -200 sented at the Surface Finishing 9 0.172 0.013 -190 Society of Japan, 87th Confer- ence, p. 253 (March 1993). Test Matrix: Standard WATTS Nickel Plus Various Addition Agents.

Plating Conditions: About the Author T 60 °C (140 °F) Robert Tremmel pH 4.0 manages the R&D CD 4.3 A/dm2 (40Amps/ft2) facility of the Udylite Division of Enthone OMI Inc., 21441 5. T. W. Tomaszewski, L. C. Process Standard 8893, Chrysler Hoover Rd., Warren, Tomaszewski and H. Brown, Motors, Auburn Hills, MI (1991) MI 48089. He joined Plating, 56, 1237 (Nov. 1969) 8. Engineering Standard, “Decorative Udylite in 1962 and has 6. J. W. Manquen, Plating & Surface Nickel Plating On Stainless Steel,” been part of the Research staff since 1964. He is a graduate of Finishing, 66, 47 (April 1979) GM 4255M, General Motors, the University of Detroit and an established 7. Engineering Standard, “Chromium Warren, MI (1988) member of the AESF Detroit Branch. Robert Plating—Decorative- Zinc Base 9. A. Fukada, M. Ozawa, M. has authored numerous publications and Alloys—Interior and Exterior,” Kamitani, T. Fukushime, T. claims more than 25 U.S. patents.

28 PLATING & SURFACE FINISHING