Studies on corrosion inhibition of steel reinforcement by phosphate and nitrite

Abstract

This work deals with the effectiveness of sodium phosphates and nitrites used as inhibitors against steel corrosion in concrete reinforcement. First, concrete pore water was simulated with several alkaline solutions. Then, reinforced concrete specimens with inhibitors added in fresh concrete mix are immersed in chloride solution. The effectiveness of inhibitors was assessed by applying electroclaemical techniques, namely Eleclrochemical hnpedance Spectroscopy (E[S), Polafisation Curves (PC), and Open Circuit Potential (OCP) measurements. The final concrete condition was analysed with Scanning Electronic Microscopy (SEM), X-Ray Diffraction (XRD) and infrared Spectroscopy (FT-IR).In solutions simulating concrete contaminated with chloride, the influence of the it~aibitors on the steel corrosion was assessed by (PC) and (EIS). The results obtained show that phosphate prevents pitting corrosion when its content is equal to chloride concentration, and that nitrite only contributes to inc~ease the value of pitting potential. Corrosion rate is reduced with both inhibitors at the different contents tested.For reinforced concrete specimens immersed in chloride solution, theft conditions were assessed by applying EIS. The results indicate that after 1 year of immersion with the two inhibitors the corrosion rate decreased. Then, aIter 3 years of immersion no influence ofinhibitors on the corrosion rate was observed. However a significant increase in concrete electrical resistance was obsewed when inhibitors were present. Visual examinations showed that all steel bars were corroded in the presence of chlorides.Results from analytical tests done on the concrete after 3 years of irranersion show that as far as the final concrete condition is concerned, the test~ inhibitors did not change the type of compounds in concrete. But the final fi'ee Chloride content remained higher than the critical chloride tN~shold. The results of FT-IR technique showed that nitrites are likely washed out of concrete during immersion in chloride solution and phosphates interfere with the equilibrium between CO3 2 and HCO3" in concrete.The main conclusion of this study is: the efficiency of the tested inhibitors decreases with time, after two years of immersion in chloride solution. I~SUMI~ ffi'aetion des rayons" X (DRX) et la Spectroscopie Infi~arouge (IR). En milieu simulant le b~ton contamin~ par les chlorures, l'influence de l'inhibiteur sur la corrosion a ~t~ dvalu~e par (CP) et (SIE), Les r~sultats ~ obtenus ont montr~ que les pho~phat~ I N T R O D U C T I O NThe application of corrosion inhibitor is a technique for preventing steel corrosion in concrete exposed to chloride environments. Several papers have been published on the effectiveness of inhibitors mixed in fresh concrete [1][2][3][4][5][6]. This paper is a complement to previous works [7][8][9], which have shown the inhibiting effect of sodium phosphate and sodium nitrite tested in simulated concrete solutions.The efficiency of these two inhibitors was assessed by using steel...

Phosphate ions as corrosion inhibitors for reinforcement steel in chloride-rich environments

“…This tendency has been observed previously by other authors [17,18]. At high [PO 4 3-]/[Cl -] ratios such as 1, phosphate acts as an mixed inhibitor, whereas at smaller ratios, phosphate becomes a cathodic inhibitor [17].…”

“…The Q t value calculated for the low frequency loop is too high to be associated with a double layer capacitance. This Q t value is therefore associated with diffusion impedance combined with a charge transfer process [17]. Moreover, the low frequency loop exhibits an angle θ with respect to the real axis that approaches 45 ○ as the frequency increases, which is in agreement with a diffusion process [43].…”

“…Some authors consider that phosphates are anodic inhibitors, being effective only in the presence of oxygen [14][15][16]. Other authors suggest that they act as cathodic inhibitors for [PO 4 -3 ]/[Cl -] ratios lower than 0.6 while they behave as mixed inhibitors for [PO 4 -3 ]/[Cl -] ratios higher than 0.6 [17,18]. Few articles have reported evaluations of the inhibitor performance of phosphate ions in simulated concrete solutions [13,14,19,20].…”

“…At high [PO 4 3-]/[Cl -] ratios such as 1, phosphate acts as an mixed inhibitor, whereas at smaller ratios, phosphate becomes a cathodic inhibitor [17]. Micrographies of the steel surface were taken after anodic polarization experiments and are presented in Figure 7.…”

Study of electroless Ni-W-P alloy coating on martensitic stainless steel

“…As pitting corrosion is important in chloride environment, the pitting potential (E p ) and the repassivation potential (E rep ), are of interest. It is to be noticed that E p is the potential where a first pit appears, when no initial pit exist before the potential scanning starts [11]. E rep is the potential below which a pit can grow and it is considered as the potential where the forward scan intersects the reverse scan [11].…”