Corrosion behaviour of innovative stainless steels in mortar

The main problem of the building sector is the limited durability of reinforcing concrete structures under highly aggressive environments due to the corrosion of reinforcements. In order to demonstrate that stainless steels are the adequate solution to avoid corrosion of reinforced concrete structures by contained chlorides, the corrosion behaviour of AISI 304 and AISI 316L stainless steels in mortar with two chloride doses is compared with the corrosion behaviour of three new stainless steels of low Ni content and reduced cost, and with the traditional carbon steel. The lowest chloride contamination (2% Cl⁻) has been used in this research to provoke corrosion in an active state of reinforcing carbon steel, whereas the highest one (5% Cl⁻) exceeds all expected levels found in the natural environment, including sea water. The new stainless steels remain in the passive state in mortar specimens with both chloride contents.     

Corrosion performance of conventional (ASTM A615) and low-alloy (ASTM A706) reinforcing bars embedded in concrete and exposed to chloride environments

Conventional reinforcing steel is used in the majority of reinforced concrete structures. In general, steel reinforcement meeting ASTM A615 specifications has been the predominant reinforcement used for these structures. Low-alloy reinforcing steel (ASTM A706) was developed and is being marketed to improve ductility and weldability deficiencies associated with the ASTM A615 reinforcement. Several State Highway Agencies have adopted the use of these low-alloy reinforcing steels. Limited research has been performed on the corrosion characteristics of the steel reinforcement meeting ASTM A706 specifications. This paper presents results from a laboratory study on the critical chloride threshold, macrocell corrosion rates, and mass loss testing for ASTM A706 and ASTM A615 reinforcing steels embedded in concrete and exposed to chloride solution. Results from this study indicate that ASTM A706 reinforcing steel exhibits lower critical chloride threshold levels and higher corrosion rates than ASTM A615 reinforcing steel when embedded in cementitious materials.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

Corrosion behaviour of low-nickel austenitic stainless steels reinforcements: A comparative study in simulated pore solutions

The use of austenitic stainless steel reinforcements is one of the most reliable methods to assure the durability of reinforced concrete structures exposed to aggressive environments, but the initial cost of the material often limits its use in practice. Nickel is one of the alloying elements that raise most the cost of the stainless steels reinforcements. In this work the corrosion resistance of low-Ni 204Cu reinforcements is compared with that of more traditional austenitic stainless steels such as 304, 304L, 316, 316L and 316Ti. Polarization tests were carried out in simulated carbonated and non-carbonated pore solutions with different chloride concentrations. Results prove the very good corrosion behaviour of 204Cu stainless steel in carbonated media and in non-carbonated, chloride contaminated media. The corrosion resistance of low-Ni type 204Cu is only meaningfully lower than that of more traditional austenitic stainless steels in very highly aggressive solutions.     

Corrosion inhibitors for chlorides induced corrosion in reinforced concrete structures

Reinforcements corrosion is the most important cause of premature failure on reinforced concrete structures. Phenomena promoting corrosion are the ingress of chlorides and the reaction of atmospheric CO₂ with cement paste. Aim of this paper is the investigation on the effectiveness of three organic commercial inhibitors in preventing carbon steel chlorides induced corrosion in concrete, since there is not yet a clear knowledge on the real effectiveness of these products. Inhibitors were added to the concrete mixture in dosage suggested by the manufacturers. Chlorides were added in the concrete mixture or penetrated from outside by “ponding” cycles with a 3.5% sodium chloride solution. The effectiveness of the inhibitors has been evaluated by long-term rebar corrosion monitoring in reinforced concrete and by rebar visual inspection after three years tests. Also solution tests were performed in order to verify the effectiveness of inhibition. Results give information about corrosion prevention ability of analysed commercial inhibitors.     

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na₂SO₄ solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i_corr and i_c increases with increasing either Cl⁻ or SO₄²⁻ concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl⁻ or SO₄²⁻ concentration.     

Corrosion performance of coated reinforcing bars embedded in concrete and exposed to natural marine environment

Corrosion behavior of mild steel plain bar embedded in concrete with 28 days compressive strength of 40 N/mm² exposed to natural marine environment at a location in the Gulf of Mannar was studied. This paper reports the results of 1-year study conducted on the reinforced concrete specimens exposed to three different levels. The performance of three different types of corrosion protection by specialty coatings to rebars, namely cement polymer composite, interpenetrating polymer network coating and epoxy coating was also evaluated periodically by measuring open circuit potential measurements and the results are discussed. Biofouling was predominant on completion of 3 months immersion.     

The effect of silane-based hydrophobic admixture on corrosion of reinforcing steel in concrete

The influence of a hydrophobic admixture based on silane on the corrosion resistance of steel reinforcement in concrete was studied. Sound or deliberately pre-cracked concrete specimens were manufactured with w/c of 0.45 and 0.80, both in the presence and in the absence of silane. The specimens were fully immersed in a 3.5% NaCl aqueous solution.The results, in terms of electrochemical measurements, visual observations, and weight loss measurements of steel reinforcement, show that silane blocked corrosion process in uncracked concrete specimens. On the other hand, in cracked concrete specimens, corrosion of steel reinforcements was unexpectedly more severe in hydrophobic specimens rather than in the corresponding not hydrophobic ones. It is believed that oxygen, which is needed to feed the corrosion process, diffuses faster in a gaseous phase through the open concrete porosity in the hydrophobic concrete, whereas in concrete without silane, oxygen diffuses much more slowly through the water filling the pores of the saturated concrete.     

Bond behaviour of corroded reinforcing steel bars in concrete

The effect of steel corrosion on bond between steel bars and the surrounding concrete was investigated for different corrosion levels. Both pullout tests and finite element analysis were used and the results from the two were compared. An electrolyte corrosion technique was used to accelerate steel corrosion. For confined deformed bars, a medium level (around 4%) of corrosion had no substantial influence on the bond strength, but substantial reduction in bond took place when corrosion increased thereafter to a higher level of around 6%. It is demonstrated that the confinement supplied an effective way to counteract bond loss for corroded steel bars of a medium (around 4% to 6%) corrosion level. The results of finite element analyses, where it was assumed that rust behaved like a granular material, showed a reasonably good agreement with the experiments regarding bond strength and bond stiffness.     

Comparison between surface and bulk hydrophobic treatment against corrosion of galvanized reinforcing steel in concrete

The effectiveness of bulk hydrophobic treatment against corrosion of galvanized steel reinforcement in concrete specimens with w/c = 0.45 and w/c = 0.75 was compared with that of surface treatment, even in the presence of cracks 0.5 and 1 mm wide in the concrete cover. In this case surface hydrophobic treatments were applied both before and after cracking as a preventive and a restorative method against reinforced concrete deterioration, respectively. The obtained results in terms of water absorption, electrochemical measurements, chlorides penetration, and visual observations carried out on reinforced concrete specimens during the exposure to wet–dry cycles in 10% NaCl solution showed that bulk hydrophobization is the most effective treatment in improving the corrosion resistance of galvanized steel reinforcements in concrete also in the presence of cracks. Surface hydrophobization is very effective just in the first few exposure cycles to the aggressive environment and when used as a restorative method which is able to cancel the deleterious effect of cracks only 0.5 mm wide.     

不锈钢在湿法磷酸中的应用

介绍湿法磷酸生产中杂质、操作条件、酸浓度、温度、搅拌、有饥物等对腐蚀的影响和材料选择的基本要素，并列出多种不锈钢的主要化学成份和几种工艺流程的材料选用表。     

Effect of steel microfibers on corrosion of steel reinforcing bars

Steel microfiber reinforcement was previously found to be successful in mitigating alkali silica reaction in concrete, an expansive phenomenon. The use of steel microfibers to mitigate rebar corrosion, another expansive reaction, was investigated. Mortar specimens with and without steel microfiber reinforcement were exposed to a corrosive environment. All specimens were prepared with water/cement ratios of both 0.40 and 0.55, cured for 28 days, and then submerged in aerated 3.5% NaCl solution. The corrosion behavior of the specimens was monitored via electrochemical measurements. Three types of electrochemical tests were performed: corrosion potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Chloride concentration measurements and microscopic analysis were performed as well. The polarization curves, Tafel, and polarization resistance measurements indicate that the steel rebar in the microfiber-reinforced mortars are more resistant to corrosion than the rebar in the control mortars, despite higher chloride concentrations. Furthermore, the steel microfiber-reinforced cement based materials have a lower electrolytic resistance. This is not indicative of a higher corrosion rate, which would be the case if it had been observed in standard mortar specimens.     

Laboratory investigation of reinforcement corrosion initiation and chloride threshold content for self-compacting concrete

Time-to-corrosion (T_i) of reinforcement in concrete and chloride threshold content (C_th) are important service life determinants for reinforced concrete structures in chloride-laden environments. In this study, the two determinants of a series of self-compacting concretes (SCC) and regular concretes were experimentally investigated. A new sampling approach for C_th determination (milling powder from corrosion active site at the rebar/concrete interface) was adopted to accurately express chloride content resulting in corrosion occurrence. It is found that the T_i and C_th follow the 3-parameter Weibull distribution. The results indicate that the corrosion initiation of rebar in concrete slabs depends upon both cement alkalinity and superplasticizer. Rebar, embedded in high alkalinity cement SCC, exhibits better corrosion resistance as indicated by the longer T_i, higher C_th and larger Weibull modulus, m. A larger Weibull modulus indicates that anti-corrosion performance of rebar in slabs is more stable and less scattered. The effects of specific superplasticizer on rebar corrosion resistance are discussed from the viewpoint of air void amount and size distribution at the rebar/concrete interface.     

Electrochemistry and surface chemistry of stainless steels in alkaline media simulating concrete pore solutions

Despite the increased use of stainless steel for concrete reinforcement in harsh chloride environments, comparatively little is known about the surface chemistry of these materials in alkaline media simulating concrete pore solutions. This work is concerned with a combined electrochemical and XPS surface analytical investigation on austenitic, ferritic and duplex stainless steels in simple NaOH and more complex alkaline concrete pore solutions. The results show that the passive films on these materials change with immersion time, the ferritic and duplex stainless steels becoming enriched in chromium oxy-hydroxide, the austenitic steel strongly enriched in nickel hydroxide. The composition of the metal layer beneath the surface film is strongly enriched in nickel and depleted in chromium and iron. The results are discussed with respect to the relation between the Fe(II) content in the films and the open circuit potential (OCP) during exposure, the film growth mechanism and localized corrosion resistance.     

不锈钢在湿法磷酸中腐蚀行为研究

本文采用腐蚀实验和电化学测试方法对不锈钢在湿法磷酸中的腐蚀行为进行了研究，并进行钝化膜的组成与结构分析。研究表明，磷酸中加入ＳＯ４＾２＾－，ｃＬ＾－，Ｆ＾－均提高合金的维钝电流密度和致纯电流浓度，加速材料的腐蚀，这些杂质共存时腐蚀性更大。湿法磷酸中存在的Ｓｉ＾４＾＋，Ａｌ＾３＾＋，Ｍｇ＾２＾＋和Ｆｅ＾３＾＋起缓蚀作用。不锈钢纯化膜主要由Ｃｒ和Ｆｅ的氧化物组成，表面形成致密稳定的Ｃｒ２Ｏ３保护膜是提     

An assessment on the mill scale effects on the electrochemical characteristics of steel bars in concrete under DC-polarization

The effects of mill scale on formation and stability of passive film on rebars in chloride-bearing concrete is investigated. Studies were carried out on some electrochemical parameters such as repassivation potential, E_rep, and passive current density, i_pass, using cyclic potentiodynamic polarization. The superficial mill scale on the rebars was naturally produced during hot-working of steel. The experiments were performed in the concrete samples with a cement/sand/water ratio 1:3:0.6. The results demonstrated that the presence of mill scale on the rebars in chloride-free concrete causes a decrease in electrical resistance of the passive film and its reparability. However in presence of chloride ions because of the severity of corrosion the effect of mill scale on corrosion of rebars is less manifested.     

Potentiostatic transient technique, a simple approach to estimate the corrosion current density and Stern-Geary constant of reinforcing steel in concrete

The linear polarization resistance (LPR) method, introduced by Stern and Geary, is widely used in determining corrosion rates of steel bars in concrete or synthetic pore solution. The major limitation of this method is that the value of the Tafel slopes, and consequently the Stern-Geary constant (B), is necessary for accurate calculation of the corrosion rate. This paper proposes a simple method for determining the corrosion current density and the Stern-Geary constant, using the results of the potentiostatic transient technique. The effect of the Stern-Geary constant on the corrosion rate of steel bar in concrete is also discussed. Results of this study show that potentiostatic transient method can be used successfully to determine the corrosion rate of steel bars in concrete without using the pre-assumed value of B.     

Comparative investigation of stainless steels used in drinking water distribution systems

Five stainless steel alloys were tested and compared to find a test method for the qualification of stainless steel used in drinking water systems. Determination of the pitting/critical potentials from the conventional cyclic polarization results was shown to be difficult when stainless steels have high Mo and Cr content. The electrochemical impedance data indicated that results received with the conventional methods do not necessarily reflect the steady state. The low frequency time constant determined from the impedance measurements showed that the anodic polarization should be conducted with very slow scan rate to get information about the mechanism. The steady state investigations were compared with the conventional electrochemical results to propose an electrochemical-based route as a standard qualification method. The qualification method was tested to be valid by a European round-robin-test.     

Concentration monitoring and performance of a migratory corrosion inhibitor in steel-reinforced concrete

Inhibitor concentration depth profiles for concrete samples treated with a proprietary migratory corrosion inhibitor (of the Cortec MCI range) are presented. The treated concrete was cored and these cores were then sectioned and crushed before being immersed in distilled water to extract the available inhibitor. The amine concentrations were quantified using an ammonium-sensing electrode and were then related to the inhibitor concentration present. The inhibitor examined, reported to contain a combination of volatile amines and amino carboxylate compounds, was found to readily diffuse through concrete. The inhibitor was subjected to a 5-year trial and found to be effective in suppressing corrosion of steel reinforcement in the presence of high chloride concentrations. The concentration profiles indicate that only relatively low concentrations of inhibitor were required to achieve inhibition in this case.     

Unified approach to localized and general corrosion of stainless steels

Localized and general corrosion of stainless steels is considered. In the case of localized corrosion, statistical parameters are deduced from the comparison of the distributions of breakdown and corrosion potential values to rank different materials, environments and designs. Cumulative frequency F is considered for breakdown (E_b) and corrosion (E_c) potential, and both are represented on a logarithm plot as E=E(log F). An extrapolated value of F (the risk H) corresponding to E_b=E_c is related to the field behavior of the material. The risk H is formally related to the corrosion current density (i) of the general corrosion approach. An analogy between the formalism of the probabilistic treatment of localized corrosion of stainless steels and that of uniform corrosion is highlighted through Monte Carlo simulation. Introducing a discretization model of the general corrosion, the time evolution of the surface roughness during general corrosion of stainless steels is reproduced in agreement with the experimental results. According to this model, the general corrosion can be treated on a similar basis of the localized corrosion in terms of discretization of corroding areas, and vice-versa localized corrosion can be treated on a similar basis of the general corrosion in terms of statistical distributions of the potential values.