Abstract
This state-of-the-art report reviews the history, chemistry/electrochemistry, mode of action and commercial availability of corrosion inhibitors for concrete. Corrosion mitigating substances like hydrophobic materials and pore blockers to inhibit intrusion of chloride salt, are not considered corrosion inhibitors in this review, and consequently, not treated.
A corrosion inhibitor can affect steel reinforcement in concrete in two ways: (1) By delaying the time of depassivation by strengthening the passive film, or (2) by reducing the corrosion rate after depassivation. An inhibitor fit into one of three broad electrochemical classes: (a) anodic, (b) cathodic and (c) mixed, depending on whether it affects the anodic reaction, the cathodic reaction, or both.
Inhibitors for concrete are either mixed into fresh concrete (mixed-in inhibitors) or applied onto the surface of hardened concrete (migrating inhibitors, MCIs) to penetrate the concrete cover.
Sodium nitrite, an anodic inhibitor, was the first inhibitor to be used in concrete (more than 50 years ago). Since then, a lot of chemical substances have been tested, of which nitrates, phosphates, amines, alkanolamines and carboxylic acids are the most investigated. Today, blends of nitrite/nitrate salts (preferably calcium) are among the most common commercial mixed-in inhibitors outside Europe.
Roughly two decades ago, amines and alkanolamines became the most common mixed-in inhibitors on the European market. These are mixed inhibitors suppressing both the anodic and cathodic reactions. In recent years, the research on the inhibiting effect of carboxylic salts and their salts in alkaline media has been intensified. Like amino groups, carboxylate groups have the potential of adsorbing to metal surfaces, thereby forming an organic layer on the entire surface, i.e. acting as a mixed inhibitor. Today, carboxylic compounds are found in commercial mixed-in inhibitors for concrete, typically as ingredients in amine-based products.
Most mixed-in inhibitors show secondary effects on the properties of fresh and hardened concrete. Nitrites/nitrates accelerate setting of fresh concrete, whereas many carboxylic acids tend to retard the cement hydration rate.
Nearly all MCIs on the market are blends of amines/alkanolamines and carboxylic acids, i.e. aminocarboxylates. Reports on MCIs deal mostly with the penetration (or lack of penetration) of the inhibitor in the hardened concrete cover. There is a debate going on whether these products really act as anticipated.
It is recommended to continue the research on carboxylic compounds, as sole admixtures, and blended with other compounds with inhibiting properties, e.g. nitrates and alkanolamines, already known for their inhibiting properties. At the same time, it is important to investigate any secondary, often undesirable, effect of an inhibitor on concrete properties in the fresh and hardened state. One should aim at counteracting undesirable secondary effects of promising inhibitors (e.g. a retarding effect) by smart blending of substances. This may also open for the development of multifunctional admixtures.
“I sometimes think that the construction industry is like the person who seeks the wonder cure: the elixir that will provide eternal life with no effort at all on their part. Wouldn’t it be wonderful if by the simple expedient of adding ingredient X all would be solved? There would be no need to worry about cover, compaction, curing or even the concrete itself. Life is not that simple...” C.D. Pomeroy (A.M. Vaysburd and P.H. Emmons, Cement & Concrete Composites, 26, 2004, pp 255-263)