___ Say “galvanized” and the concept of zinc-coated metal comes readily to mind. But that isn’t the full story. Galvanizing is not always a single, simple step, especially when it comes to coated products for automotive parts and other demanding end products. There are a variety of galvanized finishes to meet these demands in automobiles and trucks.
___ Material Sciences Corporation’s, Walbridge, OH facility specializes in various galvanized coatings for automotive, appliance, building, electronics, and HVAC applications. More than 25 years ago the company built the line to produce Zincrometal for exposed automotive body panels. In the mid 1980’s the line was up-dated to add zinc electro-galvanizing (EG) capability and a few years later the decision was made to incorporate Zinc-Nickel (ZnNi). In the early 1990s ZnNi with organic coating was commercialized for fuel tank applications. Then in 2000, they added their patented process for wide width laminating to supply the world’s auto makers with Quiet Steel® body panels.
___ The coating line at Walbridge, specifically designed for the high quality demands of the automotive and appliance industries, features a 72” combination electro-galvanizing (EG) and coating line. Here, the electro-galvanizing process can be applied to base metal one-side, two sides, and differential for exposed or unexposed products. EG produces a zinc rich layer that provides excellent corrosion protection, ductility, and weldability for pre and post paint applications. The line has 21 plating cells with over 1.2 million amps plating capacity and is considered the world’s largest coater of steel fuel tanks. “As a result of this process millions of automobiles and trucks worldwide are being protected in vital areas with EG coatings from this plant,” says Dan Hughes, sales manager.
___ For higher corrosion resistance the ZnNi coating is applied. This coating offers enhanced welding characteristics over free zinc with better corrosion protection at equal coating levels. An aluminum rich thermal set epoxy coating over the ZnNi surface is approved by major automotive producers as a fuel tank coating for gasoline, flex fuel, or bio-diesel applications. MSC also has the ability to apply the fuel tank coating over Hot Dip Galvanized (HDG) aluminized substrates. All are chrome free and environmentally friendly. They offer 15-years of corrosion protection, are suitable for deep draw applications, and produce a low cost pre-painted product.
Variety of Products
___ Other products in the MSC pipeline include ElectroBrite® and ViviColor®. These products are designed to give the customer a stainless steel surface appearance at a lower cost while incorporating anti-finger print technology to a surface that is magnetic. They consists of a ZnNi coating that is polished and painted and can be provided with or without a strippable film. It is ideal for auto consoles, radio chassis, electronic cabinets, and home appliances. “For instance,” adds Hughes, “You have the magnetic quality for refrigerator doors which display family pictures, calendars and notes. It looks just like stainless steel, which is non-magnetic, and less costly than stainless.”
___ The Walbridge facility is the only US Automotive quality EG line capable of applying zinc rich primers over EG in a single pass. These primers provide hem flange corrosion protection and eliminate cavity waxes. Walbridge gained approval by Henkel Corporation and two other major corporations for their Granocoat product. “Hem flange corrosion was a concern in the ‘70s automobile and truck production where dissimilar metals could cause galvanic corrosion,” explains Hughes.
___ EG/Phosphate and anti finger print coatings round out MSC’s product line offerings for body panels, radio chassis, and electronic cabinetry. The systems are RoHS compliant and give customers a ready to use paintable surface with enhance forming.
A Process Overview
___ Now, let’s look at the coating line in more detail. Coils of bare cold-rolled steel up to 72” wide – the widest in the industry – and typically 35,000 to 50,000 lbs. are loaded into the line. They will travel approximately one mile through the process. Since coil coating is a continuous process, the tail of one coil is resistance welded to the head of the next coil, with welds tested for integrity. The surface is cleaned of oils, mill lubes, dirt, and other contaminants utilizing high pressure sprays and hot water rinses in a counter direction of the strip flow to remove the oils and mill lubes. It is then brushed and rinsed a second time. The strip then travels through a tension leveler for shape correction. Followed by a third cleaning section where Henkel Corporation’s Parco cleaner EC76 is used to for an electrolytic cleaning of the strip. This process creates oxygen bubbles on the surface of the substrate to pullout any residual dirt that might be present. This is done to make the strip as clean as possible before the electro galvanizing process to achieve a good bond between the zinc ion and the strip itself.
___ After electrolytic cleaning the strip is rinsed, dried, and then progresses to surface activation. This is a low temperature low current sulfuric acid bath that mechanically etches the surface of the substrate to enhance the bond between the zinc and the substrate. This is followed by the strip moving through 21 electro galvanizing cells where, a sulfate bath rich in either zinc or zinc nickel is used to apply coating weights from 10-100 grams per meter squared per side. Coating weight is verified by an in-line gauge, which traverses the strip, and records results on a continuous basis. Using a man/machine interface, the process is controlled to ensure the coating meets the customer’s requirements.
___ Pretreatment starts with Henkel’s Parco® 8686 alkaline cleaner, an alkaline cleaner used in the galvanizing process. This is followed by Bonderite® 1303 complex oxide. Then, depending on the specific products being processed various treatments such as Bonderite® 1421 or Granocoat 5854 (microcrystalline zinc phosphate) are combined with tri-chrome or non-chrome sealers to produce RoHS compliant material. Other products, such as HDG aluminized, utilize Parcolene® 350C as a chrome-free rinse or Bonderite® 1325 to achieve RoHS compliance.
___ After pretreatment, the substrate proceeds to the prime coating room. The Walbridge facility has the capability of two side painting with two different paint systems simultaneously. This process is utilized for the bio-diesel automotive fuel tanks due to the corrosive nature of the fuel itself. Specifically, one paint system is designed for the exterior of the tank while the other paint system is designed for the interior.
___ The substrate then travels through the oven cure process. This process includes 190 foot of oven with multiple zones enabling proper control of the curing and solvent destruction to meet environmental requirements. The strip is then cooled with water or air quenching before it travels to the finish coating room. The finish coater also has the capability of two side painting with two different paint systems simultaneously. The strip will then travel through another oven cure process and quench before the exit accumulator.
___ As the strip exits the accumulator it goes through a two-sided inspection process where a readout of substrate thickness is recorded. Some of the customers require this measurement to make certain that the steel is within the thickness specifications. At this point coils are removed and samples are taken for final Quality Control (QC) checks.
Galvanized Can Mean More Than Zinc Alone
___ In conclusion, when you say “galvanized” you could be describing any one of a number of zinc or zinc-nickel coatings coming out of MSC, designed for handling a variety of applications where corrosion protection, quality and attractiveness, fabrication ease, weldability and a host of other capabilities for specific services are demanded. “We apply what we call ‘economies of scope’ to cover a variety of products for a variety of demanding applications and complex systems to meet whatever the customer might specify,” Hughes explains.