Thermal-sprayed aluminum (TSA) has successfully been used for corrosion protection for several decades. Coating lifetimes of more than 30 years in a corrosive marine atmosphere have been documented. When repairing damaged TSA-coated surfaces with thick coatings, the transition area between the TSA and repair coating (i.e. where protective paint overlaps the TSA) experiences rapid corrosion of the TSA.
A corrosion mechanism resembling crevice corrosion, driven by the formation of an acidic environment under the paint, causes the degradation. Although the problem is averted when TSA is not coated with thick coatings; it is difficult to avoid in many situations.
NACE International member Ole Øystein Knudsen with SINTEF (Trondheim, Norway) and his colleagues investigated the performance of different repair coatings when applied on both TSA and bare steel to evaluate their ability to decrease corrosion of TSA and protect the bare steel. Slowing down or preventing the formation of the acidic environment, or neutralizing it by buffering reactions with the paint, was expected to decrease the degradation rate.
Twelve repair coatings were selected for cyclic corrosion testing based on four different mechanisms: coatings that buffer the low pH that causes TSA corrosion; open coatings that allow the acidic environment to diffuse out; conductive coatings that enable electrochemical reactions to take place on top of the repair coating vs. the TSA/coating interface; and sacrificial coatings that corrode instead of the TSA.
All the tested repair coatings performed better than the epoxy reference coating system, which was causing the corrosion problems. Most of the coatings tested were categorized as primers with sacrificial pigments. Only one was tested with a topcoat, which improved its performance.
The researchers found that applying the repair coating edge-to-edge with the TSA, without overlap, gave good corrosion protection, even when there was a small gap with bare steel. Zinc-rich coatings, either alone or topcoated, gave the best results when evaluating them for bare steel protection as well as prevention of TSA degradation.
More information on the investigation is available in CORROSION 2017 paper no. 9369, “Repair Coatings for TSA,” by O.Ø. Knudsen, et al.