While some science and engineering subjects have enjoyed periods of feast and famine, the corrosion of materials by aggressive aqueous solutions seems to sustain its importance. In part, the relevance of aqueous corrosion to engineering systems results from the incentive of design engineers to introduce new systems into corrosive environments and to extend existing systems beyond those conditions where satisfactory materials or protective mechanisms have been previously identified. My colleague Rudy Buchheit pointed out to me an interesting economic reason why corrosion is such an important field of study. A recent cost of corrosion study estimated that, in 2002, corrosion drained about 3.1% of the GDP from the US economy. On this basis, the direct economic loss from corrosion in 2004 was $364 billion. That is about the same as the losses associated with hurricane Katrina – except that we bear this hidden loss every year. In this sense, corrosion is the largest continuing technical calamity of our time.
About 50 years ago, Mars Fontana at The Ohio State University identified eight forms of corrosion of engineering significance: uniform, Galvanic corrosion, pitting, intergranular attack, dezincification, erosion-corrosion, stress corrosion cracking, and concentration cell corrosion. Since then, crevice and filiform corrosion have also been identified. These modes are sufficiently descriptive that most scientists, engineers, and even laymen can qualitatively understand what is involved and why they are important.
For the past couple of decades, the research community has generally praised the advantages of ‘interdisciplinary research’, an approach that has always been the hallmark of aqueous corrosion studies. The thermodynamics of corrosion are based on the approach of Marcel Pourbaix, which describes the solubility of corrosion products, e.g. by a superposition of phase stability (E versus pH) diagrams for the water solvent and the relevant metal-oxygen system. Depending upon whether the metal, a solute, or a protective oxide is shown to be stable, the diagrams provide predictions for immunity, corrosion, or passivity, respectively.
Because the aqueous solutions are electrolytic conductors, the local oxidizing state in the solution is described by an electrode potential based on oxidation half-cell potentials. Thus, the chemical aspects of the subject are led into electrochemical phenomena that are the sorts of measurements used to infer corrosion kinetics. At the simplest level, the dependencies on electrode potential of the individual rates of anodic and cathodic reactions occurring on a coupon immersed in the relevant solution are measured as E versus Ipolarization curves. Faraday's Law is used to convert the measured currents into rates for the anodic and cathodic processes.
Of course, a large variety of more sophisticated transient electrochemical techniques are available to test the details of the processes. After the thermodynamic and kinetic aspects of a particular material/solution system have been ascertained, an attempt is generally made to characterize either the corroding or the protected surface using all the fancy microscopic, structural, and surface analytical techniques available in the lab. If the problem is sufficiently important, the composition, fabrication, and heat treatment of alloys and other materials (certainly plastics) to resist the specific corrosion environment might be undertaken. Additionally, protective coatings (plates or paints) or solution additives (inhibitors) might be introduced. Thus, the interdisciplinary nature of corrosion becomes quite obvious.
Let's just consider the breadth and importance of some engineering systems whose reliable performance and safety for personnel depend upon corrosion protection: nuclear reactors involving high-temperature high-pressure water, ships and oil platforms in oceans and seas, transportation vehicles (cars, trucks, and airplanes), electronic circuit boards, communication systems, buried pipelines, prosthesis body parts, food packaging, home plumbing, tanks for storage of chemicals, carnival rides, vaults for the long-term storage of nuclear waste, water purification plants, etc.
A classic corrosion disaster can occur when a phone switching center is attacked by fire and volatilized PVC insulators deposit aggressive chloride salts throughout the building. In the arsenal of protective mechanisms, the corrosion engineer can choose among several protective schemes to combat such a event: cathodic protection (e.g. impressed cathodic currents), anodic protection (e.g. galvanization, sacrificial anodes), coatings (e.g. tinplate, paints), inhibitors, change in material, etc. However, often the best engineering approach is simply to ascertain the expected service life for the use of a cheap material, e.g. steel, and then replace it on a schedule that does not interfere with or endanger the intended functioning of the system.
The interdisciplinary nature of corrosion problems and the development of viable solutions have been so important to technological development and safety that scientists and engineers from around the world historically have made important contributions.
And there seems to be no end in sight.