Global corrosion costs are estimated to be in excess of 2.2 trillion dollars per annum, or over 3% of world GDP.1 This figure is no doubt growing year on year due to this silent killer. We are not winning the war on rust! Many mature paint technologies are often adequate in terms of their efficacy in corrosion resistance, but legislation has moved to remove them from the workplace or the environment due to their deleterious health or environmental effects. This is seen with the removal of lead-based paint2 through to chromates,3 and we see the potential removal of zinc dust and possibly all heavy metals from paints and coatings. A similar story is being played out with regard to antifouling coatings, with organotin additives being removed4 and with what appears to be moves to remove copper-based antifouling agents in the future.5 The challenge for researchers and formulators is to remove these materials from paints to meet the current health and environmental legislation while still developing products that maintain their corrosion-inhibiting properties.
To help with this we have to understand that corrosion inhibitors act in many different ways. For example, some inhibit the electrochemical reactions at the cathode (e.g. magnesium, zinc, nickel) and at the anode (e.g. nitrates, chromates, molybdates, phosphates, silicates), or use an organic material as a coordinating barrier (e.g. azoles, thiols, carboxylates).6 For formulators, these traditional anode and cathode inhibitor materials come in the form of pigments to be added to the paint formulation. However, organic materials are avoided due to their incompatibility with coatings, as they are potential curing agents.