Testing Chrome-Free Pretreatments: Evaluate the System, Not the Film

For decades, hexavalent chromium conversion coatings have been the standard for measuring the performance of pretreatments on steel, galvanized steel, and aluminum-zinc coated sheet prior to painting. This standard is now being phased out. Hexavalent chromium compounds have been classified as carcinogenic to humans by the International Agency for Research on Cancer. The Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 5 µg/m³, as an eight-hour time-weighted average, while the Environmental Protection Agency (EPA) is currently working on rules for per- and polyfluoroalkyl substances (PFAS) discharges from chrome-finishing operations, which include chromate conversion coating. For coil coaters, home-appliance and building-product manufacturers, and finishing job shops across the United States, one fact is becoming increasingly apparent: a growing number of companies are exploring chrome-free pretreatments, and many are qualifying a non-chromium product for the first time.

Interestingly, many of these trials are not thwarted by chemistry – in my experience supporting pretreatment trials on production lines in several countries, one of the most common and avoidable problems I encounter is testing a good product in a manner it was never designed to withstand.

A Test That Measured the Wrong Thing

Recently, a customer in Southeast Asia undertook an evaluation of a chrome-free pretreatment for aluminum-zinc coated steel (GL). After subjecting the pretreated panels directly to a neutral salt spray test (ASTM B117) for 24 hours (without the primer and topcoat the product was intended to bond with), the panels turned black and the result was reported as a failure.

However, the product did not fail; the test did. Pretreatment is intended as part of, but not the end of, a paint system (see Figure 1). A conversion film that is far thinner than any primer or topcoat is simply not designed to, and should not be expected to, resist prolonged salt-fog exposure. The blackening was a sign that the test did not match the product’s design, not that the product was defective. What the customer should have done, instead of requesting reformulation, was to reconfigure their evaluation to consider the system.

Figure 1. Schematic cross-sections, not to scale. (a) This shows the system that the pretreatment is designed for. (b) The configuration customer tested in the scenario described: the pretreatment was exposed solely to salt fog.

Pretreatment and Post-treatment Are Different Products

Much of the confusion stems from the conflation of two types of products (Figure 2).

A pretreatment is a thin layer or dried-in-place film that is applied to clean metal surfaces prior to successive layers of primer and topcoat. Its job is to enhance adhesion between the metal and primer, and to provide additional corrosion protection at the local interface. The barrier and weathering protection are provided by the primer and topcoat. Hence, the appropriate sequence for a pretreatment trial is to begin with degreasing, followed by pretreatment, primer and topcoat, before proceeding to performance testing.

A post-treatment (either organic or inorganic for unpainted galvanized or aluminum-zinc coated sheets), on the other hand, is the outermost layer and is designed to withstand environmental exposure on its own. It is perfectly acceptable to expose a post-treated panel directly to salt spray, as it is intended to withstand such conditions. However, if the wrong criteria are used to evaluate a pretreatment, the results can be misleading, turning otherwise satisfactory products into failures on paper.

Figure 2. Pretreatments and post-treatments have distinct roles and require different testing methods.

Why Chromate Habits Can Mislead

Expectations around chromate are understandable. Chromate conversion coatings are inherently forgiving – the soluble hexavalent chromium contained within the film can migrate to areas with defects and suppress corrosion, a self-healing behavior that has been widely documented in corrosion literature. Alternatives that do not contain chromium, such as zirconium- and titanium-based conversion coatings, are being developed to provide similar levels of corrosion resistance and paint adhesion, while avoiding the health and environmental implications of chromium. However, without the active inhibition that chromium provides, these products tend to rely more heavily on the system and consistency of processing. As a result, tests that might have worked well with chromate may not yield consistent results with a chrome-free product.

Control the Variable Before Trusting the Result

Even the best-laid tests can be foiled by the incorrect amount of pretreatment on the panels. The coating weight – which we usually measure in mg/m² for a particular marker element or the whole film – is critical to understanding the adhesion properties and resistance to corrosion. When comparing products or a trial product to a control, it is critical that coating weights are checked and matched, which is what lends the results their significance.

While X-ray fluorescence (XRF) is the technique of choice for determining the coating weight (mg/m²), the accuracy of an XRF measurement depends on its calibration. Even two products that contain the same marker element (e.g., silicon), will typically require different calibrations. This is because the amount of that element in the dried film can vary significantly depending on the formulation. Using the calibration for one product on another can skew the measured coating weight (see Figure 3). One way to deal with this is to develop a calibration for each product using panels coated with a series of dilutions (0, 25, 50, 75, and 100 percent), under the same conditions.

Figure 3. An illustrative schematic not based on any empirical data. Two substances where both have silicon as an indicator element can produce an identical XRF reading even though they have different coating weights; hence, each products needs to be calibrated separately.

For roll-applied, dried-in-place products, determining the bath concentration that will yield a certain coating weight is straightforward, using a simple mass balance:

C = W ÷ (t × ρ × S)

where W represents the target dry coating weight in g/m², t is the actual wet film thickness applied in µm, ρ is the density of the working solution in g/cm³, and S is the nonvolatile (solids) fraction of the concentrate. Hypothetically, if W = 0.10 g/m², t = 5 µm, ρ = 1.0 g/cm³, and S = 0.20, we see that the required concentration C is equal to 0.10, or a 10 percent solution. It is important to note that the wet pickup can vary depending on the coater and line; therefore, it is best to measure t rather than rely on theoretical values. A simple spreadsheet based on this relationship can be invaluable to a trial team in setting the bath before running the first panel.

A Practical Qualification Sequence

The following sequence keeps the evaluation focused on what will go into service (Figure 4):

1. Define the system. Clearly outline the substrate, pretreatment, primer, and topcoat as they will be used in production.
2. Verify cleaning/degreasing. A pretreatment will not work on a dirty surface, so confirm cleanliness. One way to do this is using a water-break check (ASTM F22) prior to application.
3. Confirm coating weight using a calibration specific to that product in the technical data sheet (TDS).
4. Apply the complete paint system under curing conditions that mimic those in production. Obtain bar coater details from product suppliers for laboratory testing, or use roller equipment, if available.
5. Test the entire system. Common tests include neutral salt spray (ASTM B117) on scribed panels, measuring scribe creep according to ASTM D1654, adhesion using the tape test (ASTM D3359), humidity resistance (ASTM D2247), and for coil products, T-bend flexibility (ASTM D4145), always in accordance with the customer’s specifications.
6. Conduct a control. Process the current product (either chromate or an existing chrome-free system) along with the trial product under the same conditions, so that any differences can be attributed to the pretreatment.

Figure 4. The recommended sequence.

If investigating the behavior of a pretreatment alone, for instance to examine film formation, it can be tested separately. However, ensure that the results are reported accurately: it is a film check, not a performance evaluation.

Getting the Transition Right

Transitioning from hexavalent chromium to chrome-free pretreatments presents a qualification and a chemistry challenge. If a good chrome-free product is rejected due to a faulty test, it sets the transition back. Conversely, if a substandard product is accepted due to a faulty test, it undermines confidence in the alternatives. While defining the system, managing coating weight, and testing what will actually go into service are simple practices, they are critical for U.S. manufacturers to make the transition confidently.

Figure 5. Panels exposed to salt spray: (a) Cr Pretreatment after 24 hours; (b) Non-Cr Pretreatment after 24 hours; (c) Complete System (Cr and Non-Cr work equally well).

About the Author

Syed Hasnain Abbas is a Manager, Technical Development, at NIPSEA Chemical Korea, part of the Nippon Paint Group, where he develops and supports degreasers, pretreatments, post-treatments and other functional coating technologies for galvanized steels and other metallic substrates, including production-line trials. He holds an M.S. in materials science and engineering from Seoul National University and has reviewed multiple manuscripts for the Journal of Alloys and Compounds. Email: [email protected]

Sources                                                                

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