Mercury’s Powder Coating Stands Up to Tough Marine Environments
Posted on Thursday, May 30, 2019
Mercury Marine is the world’s largest producer of recreational outboard motors, and their iconic Phantom Black engines are recognized around the world for not only their performance, but also the high-gloss black that holds up extremely well to harsh marine environments.
Recent additions to the color palette at Mercury have expanded the options that have proven popular with consumers. The use of powder coating has been a vital component of the esthetic performance of Mercury’s expanding product portfolio.
The use of powder coatings at Mercury facilities date back to the mid-1990s at the plant in Stillwater, OK, which manufactured MerCruiser sterndrives. The initial powder coating had been developed to serve various industries, and it provided the desired visual appearance for the sterndrive market. The powder coatings offered numerous advantages compared to the traditional high-solids liquid paints that were commonly used at the Fond du Lac, WI, campus. The powder coatings emitted almost no VOCs (volatile organic compound) and very low HAPs (hazardous air pollutant) compared to the liquid systems in use. This was very advantageous for the emissions regulations of the plant, which used liquid paint at the time. However, Mercury Outboards decided that it needed a marine-specific powder coating to achieve the largest benefits for consumers.
Mercury started to develop this marine-specific formulation by working closely with chemists at a chosen powder coating supplier. Numerous meetings were held to explain the testing and approval protocols, the qualities that the coating needed to achieve, as well as the enhancements that were desired over the existing product. The initial development work resulted in two variations of formulation chemistry that were designed to meet both the corrosion protection and the esthetic requirements of the product. These formulations passed the laboratory testing by the coating supplier, but that wasn’t enough; Mercury then subjected the coatings to the harsh testing of the saltwater tide range test.
A proper description of the saltwater tide range test is required to obtain an appreciation of the extent of effort and time that Mercury invests to validate coatings used on its products. Mercury has been performing this tide range testing for approximately 50 years, so an extensive database of knowledge and performance exists as a reference. First off, the test coupon is not just a flat panel, as is customarily used for testing coatings. Instead, an actual engine component, a gearcase, becomes the “test coupon.” Figure 1 shows a gearcase that is used for the testing with the accompanying inspection locations identified.
An actual gearcase is used as the test coupon for the following reasons:
• The part is made from the exact same alloy as parts that are used in daily production for a wide range of components.
• The part contains the design features that are present on numerous components representing the various features that must be coated on a daily basis.
• The part experiences production finishing operations similar to many parts that are produced on a daily basis.
• The part can be hung on production equipment to have the coatings applied in a production environment to provide the most realistic validation of the coatings’ performance when in production.
The gearcase is then sent to the Mercury Environmental Research Center (M.E.R.C.) on the Gulf Coast of Florida for a two-year tide range test. The tide range is pictured in Figure 2. This picture depicts the gearcases up on the railing for their bi-weekly inspection at each of the sites listed in Figure 1. Each test configuration, which represents different combinations of coatings, is repeated in triplicate and randomized on the tide range rack.
Under normal testing conditions, the gearcases are suspended by the ropes seen in Figure 2, and then Mother Nature does the heavy lifting. When the tide comes up, the gearcases are submerged. When the tide goes out, the gearcases are exposed to the air and can dry out. This cycle is repeated with the tidal cycles on a daily basis for the two-year duration of the test. However, Mother Nature adds additional complications to the testing. Since this test uses ocean water, there is an abundance of bio-life present that must be accounted for. Since bio-fouling likes to attach to stationary objects in the water, the gearcases have to be cleaned on a weekly basis. The gearcases are removed from the water and hand washed with a wool mitt to remove the barnacles and other bio-life in its larvae stage before it can destructively attach to the surface of the gearcase. Figure 3 shows what happens to a control sample from a bio-fouling test conducted on the tide range after four months of exposure if this procedure is not performed!
The data that is generated is plotted on a graph with Corrosion Units versus Weeks of Exposure to determine the performance of a coating system. Figure 4 shows a typical graph depicting a full test matrix for a given test. The corrosion-units number is the compilation of the measured amount of corrosion at each of the 27 sites on the surface of the gearcase for all three gearcases combined. The plotting of corrosion units versus weeks of exposure creates the graph below. Coating systems represented by lines that rise quickly and continue to grow in corrosion units are undesirable coatings for a marine application (see the left side of the graph in Figure 4). Since there are extensive man-hours involved in the weekly cleaning and bi-weekly rating of the gearcases, components that are very poor performing are removed prior to completion of the test period. Past research has proven that coatings that exhibit this type of behavior will never “recover” and go into a minimal corrosion growth phase.
As depicted in the graph, a very large range of coating performance is available, even from suppliers that claim to understand the needs of the marine environment. Many coatings meet performance criteria on flat panels in laboratory testing condition to a given specification, but when tested in the environment, processed with production equipment on production type parts, the results speak for themselves.
The two versions of the powder coating were processed through the production systems on the test gearcase. These samples were then placed into the tide range along with control samples and numerous other coating systems for evaluation. After one and a half years of the two-year cycle of tide range testing, it was determined that one of the development powder topcoats outperformed the other for corrosion protection characteristics. At this time, it was decided that a second set of the gearcases with the preferred powder composition would be produced and the testing cycle started again. These tests were carried out for the full two-year test cycle. All of this testing is done in conjunction with control samples (from powder types that are currently in use and/or have been used in the past) that are coated through the standard production process to use as a comparative baseline. It is important to conduct the baseline testing because, unlike a corrosion test cabinet in a laboratory, Mother Nature does not run an identical cycle every day or every year. By running various control samples and comparing that performance to the same coating system controls run many times over different time periods, a comparison can be made with a high degree of confidence.
The powder coating formulation that eventually went into production for the Mercury outboard motors was developed and tested over a period of about five years. While this may seem like an extensive amount of testing for only a “coating,” the finish is one of the first things that a consumer will notice when looking at a Mercury engine in the showroom or on a boat. It is the responsibility of the coating engineers and manufacturing personnel to provide the consumer with a coating system that will continue to deliver those stunning characteristics for the life of the product.
Chris Misorski is technical advisor materials and coatings for Mercury Marine.