Part Buyers Authority Issue #2

24 PART BUYERS A U T H R I T Y Contact: BOB BILTZ [email protected] Barrier Layer (Bright Nickel) Plating a single layer of nickel, such as bright nickel, over an aluminum or steel item will provide corrosion resistance to the part only so long as the barrier is intact. Duplex Nickel (Bright Nickel over Semi-Bright Nickel) By plating two layers of nickel, with the top layer being more active (i.e. less noble) a new level of corrosion performance is reached. This is generally accomplished by plating bright nickel over semi-bright nickel. Once the initial bright nickel barrier is penetrated, the corrosion spreads laterally into the more active bright nickel layer instead of penetrating the less active semi-bright nickel layer. Microporous Nickel The addition of a microporous nickel layer over a duplex nickel deposit further increases the corrosion resistance of a plated wheel. Instead of penetrating a single corrosion site corrosion potential is distributed into many smaller sites. The result is that instead of a single large corrosion site, there are hundreds or thousands of smaller sites, all of which corrode much more slowly. High Sulfur Nickel The addition of a high activity nickel layer between the bright and the semi-bright nickels result in even better corrosion protection. Once the nickel barrier is penetrated, corrosion occurs laterally in the active high sulfur layer, thus slowing penetration of the corrosion to the substrate. Today’s real chrome is the true choice for the best appearance, the best performance and the best value for the discerning consumer. Proper pretreatment is critical to ensure maximum adhesion, to optimize the appearance, and to control the performance characteristics. One critical aspect of any industrial finishing process is its ability to maximize productivity regardless of the condition of the base substrate. KCW partner and supplier, MacDermid, has a unique process cycle that maximizes the adhesion to the base aluminum, while simultaneously offering a high degree of resistance to substrate porosity, two of the major causes of rejects in OEM wheel plating. There are several successful methods for processing A356 alloy, but the pretreatment process KCW uses is unique. It is called the “Dual Etch” cycle. There are several advantages to this cycle: • There is a significant increase in adhesion compared to conventional aluminum pretreatment processes. • It does not utilize nitric acid. • The cycle is fluoride free. • The process is cyanide free. In general, the purpose for plating a wheel is to create a hard, corrosion resistant cosmetic coating that provides an attractive finish, provides an abrasion resistant coating, and protects the wheel from corrosion damage. Plated wheels are commonly referred to as “chromed wheels”. This term misrepresents what is actually an exceptionally complex plating system, where a series of coatings, each applied for a different purpose, are applied to meet the necessary quality requirements. Typically, plated OE wheels have six or seven layers of plating. The basic plated layers are below: 1. Nickel strike 2. Acid copper 3. Semi-bright nickel 4. High sulfur nickel 5. Bright nickel 6. Microporous nickel 7. Chrome Each layer serves a specific purpose, and to maximize the overall appearance and performance of the wheel all layers need to be applied correctly. The strike layer is to provide adhesion of subsequent plating processes. Acid copper will level out minor surface imperfections in the aluminum casting, provide a soft deposit which can be polished, and increase the corrosion performance of the wheel by providing a copper barrier coating on the wheel. The nickel layers will be described in detail below. The chrome is applied for appearance, to prevent oxidation of the nickel and provide abrasion resistance. The nickel layers have different properties. Specifically, each layer has a different electrochemical potential. These differences in electrochemical potentials are the main factor in the corrosion resistance of the wheels. The potential differences between the layers can be measured and specified. The most common method is the STEP test, a test which measures both the thickness of the nickel layers and the electrochemical potential of each layer. STEP stands for Simultaneous Thickness and Electrochemical Potential. The multiple layers of nickel are the primary reason for the tremendous corrosion resistance seen on today’s high-quality wheels. These anti-corrosion mechanisms are: