Controlling Quality of Zinc Flake Coatings on Steel Fasteners

Q: How do you check the cleanliness of fasteners when processing through an alkaline degreasing process?

A: The quality of zinc flake coatings depends on good adhesion and bonding to the substrate of the fasteners. The aqueous cleaning process must have enough cleaning stations (for example, alkaline cleaners and alkaline descalers) to properly remove the different types of oils, soils and heat treat scale. A good flowing water rinse after the alkaline cleaner and descaler is key to achieving a high-quality, clean fastener.

To verify the cleanliness of the fasteners, a few quality checks are utilized. Grab a few fasteners after the cleaning process to test for oils and soils. One method for checking cleanliness is surface tension of the substrate by using a test ink method. A good value for alkaline-cleaned part cleanliness should have a surface tension at or above 38 mN/m for a positive result. When applying the test ink, the results should be a solid line on the substrate, if there are any breaks in the line, then the substrate still has oils remaining on the fastener.

Another method is the water break test. Acquire samples and then use a 1 mL disposable pipette to place a droplet of water onto the surface of the fastener. If the water droplet breaks form (wets out), the parts are acceptable. However, if the water droplet maintains form (similar to water drops on a freshly waxed car), then the fasteners still have oil contamination and must be processed through the pretreatment system again.

Next, parts should be tested for alkaline dragout. Place fasteners into a beaker of deionized (DI) water and add 4-6 drops of phenolphthalein into the DI water. If the water turns pink in color, alkaline residue remains on the parts and must be reprocessed using good rinsing.

If allowable by the fastener manufacturer (and only after the fasteners have been processed through the alkaline cleaner, descaler and receive approved cleanliness inspection by Quality), then the next step in the cleaning process is to mechanically clean the fasteners to remove heat-treat scale and other debris. This is done by utilizing a shot blast unit which impinges the fastener with either a spherical or cut-shot media.

After the mechanical cleaning has been completed, acquire samples for the following checks. A tape test is used to determine if the substrate is free of any residual dust left behind after the blasting process. The tape is adhered to the substrate and then pulled off. If the tape exhibits dust, the parts have not been properly processed.

The next test method is immersing the fasteners into a copper sulfate/HCl solution (CuSO4). If the fasteners exhibit a uniform salmon color, the parts are cleaned adequately. If the salmon color has dark areas, the parts still have heat-treat scale remaining and another blast cycle is required.

Q: After the pretreatment process has been completed, how do you ensure that the zinc flake coatings are being applied correctly?

A: There are multiple steps to ensure that the processing of the zinc flake coatings are being processed correctly. The zinc flake coating must be properly mixed to ensure that all of the solids have been properly dispersed into solution. Once these solids have been mixed and the material has a homogenous consistency, the following quality checks need to be completed.

Viscosity is checked by using the supplier’s recommended viscosity cup; temperature by using a certified thermometer; and testing of the percent solids by using a lab oven or a moisture analyzer. Always refer to the supplier’s TDS for the proper procedures and operating ranges. Once these checks are passed, then the coating procedure can start.

Zinc flake coatings need to be filtered on a monthly basis, at minimum, to ensure that all contaminates are removed to maintain a high level of quality. The filtering processing can be achieved by using specific sized micron screens (as prescribed by the supplier) as well as magnets to remove any metal filings. Verify with the supplier to ensure that magnets can be used.

Next, verify that the processing recipe is correct for size and type, and configuration of the fasteners are correct to allow for the proper layer thickness of the coating. Too little or too much coating thickness applied can affect the overall performance and quality of the fasteners.

To ensure that the best possible zinc flake coated parts are supplied to the customer, the recipe should include the following:

• Basket weights should be maintained to ensure proper part movement during the spinning process.
• Ensure baskets are no more than ²/₃ full as volumes and weights vary from part to part.
• Next, curing. Proper precure, cure and line/belt speeds are important. Vary based on size and type of fasteners processed, and determine time at temperature to achieve part metal temperature (PMT), refer to the suppliers’ technical data sheets (TDS).
• After curing, thickness, adhesion via tape test, scribe tape test and a cure check using a chemical rub are utilized to ensure that the zinc flake coating meets the requirements as called out by the supplier and customer.

The Rule of Thumb is to utilize the 3 Cs — cleaning, coating and curing:

• Cleaning — ensure that the process leaves the substrate free of oils, soils and scales.
• Coating — ensures the coating parameters coincide with the TDS.
• Cure — ensures the TDS is followed.

Following these three rules will ensure the processing of zinc flake coatings will achieve the desired results each and every time.

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Scott McEuen is North American business development manager of zinc flake coatings at Atotech. Visit Atotech.com.