Ion-Exchanged Silica Anticorrosive Pigments: A Review and Recent Developments

By Tim Fletcher

Cost reduction and health, safety, and environmental issues are primary drivers of change in the coatings industry today. The users of all kinds of metal products are not only concerned that the products they buy will be long-lasting and the costs of maintenance minimal, but also that the materials used are damaging neither to human health nor the environment. The coatings manufacturer is thus continually faced with the need to provide coatings with ever-greater durability and protective properties to weathering and corrosion processes, without resorting to harmful substances. Corrosion processes on metal surfaces can have a substantial influence on the economy due to the degradation and damage caused.

The exact value of direct corrosion costs is hard to determine, but according to some sources, is estimated to be about 3% of the GDP.* This economic consideration is therefore a highly important aspect of new and improved anticorrosive technology. The corrosion-inhibitive substances currently used in coatings are for the most part based upon species known to act as corrosion inhibitors for metals in aqueous solutions.1-3 Typical examples are chromates, molybdates, tungstates, vanadates, phosphates, phosphites, polyphosphates, borates, metaborates, nitrites, silicates, ferrites, and nitrophthalates as their various metallic salts with polyvalent metals such as zinc, magnesium, calcium, strontium, barium, aluminum, and—more historically—lead.

Pigments containing aluminum ions may reflect a salt with the corresponding anion, or may reflect the corresponding anion adsorbed or exchanged onto an alumina substrate. In some cases, multiphase pigments composed of more than one component have been developed.

A stochiometric excess of the metallic component is also often employed, implying, for example, in the case of zinc-based compounds, the existence of zinc oxide or zinc hydroxide as a separate phase. Metal oxides such as red lead and zinc oxide, as well as finely divided metals such as zinc, are widely known to promote the anticorrosive properties of coatings, as are laminar pigments such as micaceous iron oxides and aluminum flake.3-5 In alternative approaches, organic substances, such as succinic acid derivatives of mercaptobenzothiazole, have been developed as corrosion inhibitors for coatings.6 Ion-exchanged silicas, particularly those based on calcium ions, are also well known as inhibitive pigments.⁷