Q: What is the difference between vapor phase and calcium nitrite type corrosion inhibitors for steel reinforced concrete?
A: Vapor phase corrosion inhibitors (VCI) are used to inhibit rebars corrosion in reinforced concrete structures. VCI containing formulations are added to the concrete mix at the rate of about 1quart per cubic yard, or are applied to the surface of hard concrete at the rate of about 100ft2/gal. There are two basic types of VCI active ingredients, amino alcohols and amine carboxylates.
Calcium nitrite containing formulations are added to concrete mixes at the rate of about 4 to 6 quarts per Yd3. Calcium nitrite is manufactured by W. R. Grace and sold as a 30wt% water solution as DCI. There are also many private label versions of the DCI formulation.
VCI type of rebar corrosion inhibitors when added to a concrete mix or to the surface of hard concrete migrate in the vapor phase through the cement phase micro pores and deposit on the rebar surface where they furnish corrosion inhibition at both the anodic and cathodic sites (Kepler 2000). The performance of VCI inhibitors have been measured using ASTM G 109 when used as an admixture and as a surface applied formulation. The fact that they will reach the rebar level at 1 to 3 inched of depth when applied on the concrete surface is the primary evidence of their ability to migrate in the cement phase. They are not consumed in the course of the corrosion inhibition mechanism, and do not have any impact of the concrete curing process or ultimate physical properties. There are several suppliers of VCI type products for uses as concrete admixtures or surface application. Two are made by Surtreat Holding, LLC and are sold as TPS XII (surface applied) and TPSXV (admixture applied).
Calcium nitrite containing formulations have been demonstrated to perform as rebar corrosion inhibitor only in the admixture use mode using ASTM G 109. There is no reported evidence of their performance as a surface applied formulation that would indicate their ability to migrate in the cement microspores. The generally accepted corrosion inhibition mechanism involves the consumption of the nitrite ion to function as an oxidizing agent to form a stable ferric oxide surface at the anodic corrosion function sites (Berke 1985). This will result in the eventual consumption of the calcium nitrite at or near the rebar surface and it is unlikely that it will be replenished through migration. These limitations in the performance of calcium nitrite containing inhibitors products is compensated for by the relative high level of addition to a concrete mix, 6 quarts in comparison to 1 quart per Yd3 for the VCI types. Calcium nitrite acts as a cement curing accelerator. This can be beneficial if used in a quick curing patching type of mix, but is detrimental in a large continuous pore where the build up of heat from the curing reaction would be detrimental, and require addition of a retarder. There are some conditions under which calcium nitrite can cause cement degradation (Hope 1995).
The effective life of these two types of corrosion inhibitors when used as admixtures can depend on how long it takes for the conditions for corrosion the reach the rebar level and the extent that a sufficient concentration of the inhibitor active ingredient is maintained at the rebar level with respect to the corrosion accelerating chloride content.
Robert A. Walde, Chief Technical Officer, Surtreat Holding, LLC contact
Surtreat Holding, LLC 437 Grant Street, Pittsburgh, PA 15219; 412.281.1202, official website www.surtreat.info
Max Merzlikin, Surtreat Holding, LLC 2015