Mechanical & Chemical

Corrosion

Corrosion occurs under the correct environmental conditions, and usually involves the metal substrate, salts and moisture in the presence of oxygen. Corrosion will cause the delamination of the coating, usually first apparent as blistering of the coating or filiform corrosion.

Processing Issues

Parts are rusting on line between chemical pre-treatment and powder coating.  Sometimes referred to as 'flash rusting'.

  • Potential Cause

    The chemical pre-treatment line is not chemically balanced; leading to too much deposition of the chemical.  Surfactants and soils remain, limiting the cleaning and pre-treating process.  Typical signs that rinsing is inadequate will be strong staining of the metal surface.

  • Solution

    Test all stages in the preparation process.  In particular, test whether the phosphate coating weight is too low or high; check that there is enough detergent to remove the oils from the manufacturing process; inspect rinse tanks and ensure they are clean and not depositing oils after the chemical cleaning stage – also check that chemicals from the first stage are not being carried over into the rinse tank; inspect the spray nozzles – ensure that they haven’t been knocked by parts travelling through the cleaning process and that they are clean and adjusted for maximum spray force and coverage.  Check that parts are hung and orientated to achieve maximum coverage.  If in doubt, contact your pre-treatment supplier.

  • Potential Cause

    Dry off is occurring between chemical stages.  As the chemicals have not yet been rinsed, dry-off will make the chemical solution on the metal surface more concentrated, increasing the chemical attack.

  • Solution

    Allow sufficient time for adequate drainage but not dry off between stages.  Reduce temperatures.  Utilize fog nozzles and adjust nozzles for maximum coverage.

  • Potential Cause

    Final dry off is too slow and flash-rusting is occurring.

  • Solution

    Increase dry off oven temperature

  • Potential Cause

    Shot-blasted parts are left too long before coating, or the environment is catalysing the flash rusting.  Acidic or humid environments will adversely affect the length of time a shot-blasted component will

  • Solution

    Take precautions to store the shot-blasted parts in a warm, dry environment prior to coating.  Reduce the time between blasting and coating.

In situ

Parts are rusting prematurely in situ.

  • Potential Cause

    The most likely cause is poor pre-treatment application or a poor choice of preparation for the substrate.  It is important that the correct system of preparation is chosen before powder coating application, taking into account the substrate and the conditions that the coated piece will be exposed to.  Each substrate will have its own particular pre-treatment system – there is no ‘one size fits all’ solution to chemical pre-treatment and surface preparation in general.  Environments are classified in ISO 12944-2 from C1 to C5 (lowest to highest corrosive environment)

  • Solution

    On-site rectification or replacement of coated parts.  To prevent recurrence, use a more appropriate system of preparation to ensure the best key of powder coating to the surface.  Make engineering steps to eliminate areas that cannot be painted, or that will trap pooled water.  Take precautions to avoid or remove sharp corners and edges where coating weight will be very low.  Consider using a two-coat system to build film weight around the corners.  For very aggressive environments, ensure that even non-significant, or hidden, surfaces are coated.  If in doubt, test the preparation method using a salt fog cabinet.

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