Methods to Improve the Corrosion Performance of Microporous Nickel Deposits
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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