Electrocoat Q&A: Best Testing Practices

Q. Could you provide an outline of the best practices for testing continuous verification of overall performance in an automotive electrocoat system?

A. Continuous conformance and validation of automotive OEM specification requires protocols that involve testing the chemical and physical properties of the washer and processes, as well as destructive accelerated physical and corrosion testing of the finished parts and components. Automotive quality systems require that this physical and corrosion testing be performed by accredited laboratories.

Testing of the physical and chemical parameters of the washer and processes are typically established by the supplier or suppliers of the pretreatment chemicals and electrocoat. These tests cover parameters such as pH, concentrations, conductivity, temperatures and others. The tests have been previously established by the suppliers and users based on research and development, and the history and experience with the specific technologies. The tests ensure that the washer, process and chemicals are operating under process conditions known to provide best corrosion protection.

Although these tests ensure process stability and control, they do not validate the intended final outcome of the process as measured by functional performance of the finished parts and components. As suggested by former President Reagan, “Trust but verify.” Other testing must be included in the quality control plan to verify and validate continuous corrosion performance. The validation must be performed under the test procedures and guidelines specific to each OEM.

Best in class ecoat systems typically employed five common test families to verify continuous conformance: film thickness, adhesion, water immersion, cure and some accelerated corrosion cycle test. Some systems use a sixth family test that either incorporates flexibility (conical or cylindrical bending) or impact resistance (sand chip or stone).

The testing can be performed on a weekly, monthly, semi-annual or annual basis depending on the type of system, type of parts and OEM minimum requirements. The tests can be done in-house or in outside accredited laboratories.

Film thickness is typically performed using electronic equipment with capabilities for each specific metal substrate. This equipment is very reliable in determining the overall film thickness of the ecoat.

Adhesion testing may be performed using one of the three testing procedures most widely used: X-cut, straight line and grid adhesion tests. Typically, adhesion tests using the grid procedure provides the best qualitative correlation and quantitative test results.

Water immersion testing may be performed at ambient or heated conditions. Some tests are conducted using tap water, deionized water or mixtures of salt and water.

Immersion tests using heated conditions provide the most severe test conditions in short periods of time, but do not have good correlation with field results. The boiling water immersion tests, the extreme in immersion temperatures, produce extreme adhesion test results in 2- to 5-minute tests. Water immersion tests for 24–48 hours at ambient temperatures have shown better correlation to field results than boiling water tests.

Cure tests are typically done with solvent rubs over the cured ecoat or with lab equipment that measures transition or gel temperature of the film. Because of the complexity and expense of the lab equipment needed to perform these tests, solvent rubs are typically used.

The type of solvent employed and the number of rubs are determined by the e-coat supplier. The test is performed using a cloth soaked in solvent such as MIBK, MEK, acetone or xylene. Transfer of the cured e-coat material to the cloth after rubbing is a clear indication that the ecoat film did not fully crosslink and cure during the curing cycle.

Accelerated corrosion tests may include the salt spray (ASTM B-117) or other specific cyclical tests. The salt spray ASTM B-117 test is widely used by industry throughout the world as it has been around for many years. This test provides a fair correlation in moderate field environments when used with steel substrates, but shows poor correlation to field results when used with other substrates such as zinc coated steels or aluminum.

Many OEMs have moved away from salt spray ASTM B-117-only tests to using other accelerated cyclical tests. These tests were developed by many national and international organizations, governing bodies and specific OEMs.

Accelerated tests involve daily or weekly test cycles that may include humidity, salt spray in combination with heat and dry periods. Due to the cyclical nature, these tests have greater correlation to severe field environments with respect to corrosion performance. Corrosion performance is typically measured by cosmetic appearance, creep or corrosion weight loss. Some specific cyclical corrosion tests also incorporate impact testing to the cycles to better duplicate field impact conditions.

The SAE J2334 is an accelerated cyclical test used throughout the world by many companies to validate corrosion performance. This laboratory cyclical corrosion test is based on a field-correlated design of experiment conducted by the SAE Automotive Corrosion and Prevention Committee and the Auto/Steel Partnership Corrosion Task Force. It has the highest correlation to severe field exposure results with respect to cosmetic corrosion and weight loss.

Continuous verification of conformance to applicable specifications is a significant part of the overall product quality plan.

Originally published in the December 2015 issue.