Quantitative characterisation of steel/cement paste interface microstructure and corrosion phenomena in mortars suffering from chloride attack

Chloride ions constitute one of the deleterious agents that may cause or promote corrosion of steel reinforcement in concrete. The influence of chloride ingress on mortar microstructure (including microstructural alterations of hydration products and of pore structure) has been studied by the authors on the basis of cross-section image analysis of reinforced mortar specimens [D.A. Koleva, J. Hu, A.L.A. Fraaij, N. Boshkov, Influences of chloride ions on plain and reinforced mortars, investigated by combined microstructure and electrochemical approaches, Paper 315, Eurocorr 2005, September 4–8 ’05, Lisbon, Portugal]. This paper specifically pursues exploring the morphological aspects and chemical compositions of the corrosion products deposited on steel surface. For this purpose, scanning electron images (SEM) were taken on the cylindrical surface of steel reinforcement and also on the corresponding positions on cement paste surface for visualisation and microstructural investigations of corrosion products. In addition, energy dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD) are employed for quantitative characterisation of the corrosion products at the steel–paste interface. Electrochemical impedance spectroscopy (EIS) is used to estimate the corrosion current and corrosion rate for the reinforced mortars. The EIS measurements are in good agreement with the microstructural observations and quantitative analysis of various corrosion products. The combination of electrochemical measurements with quantitative microstructure analysis of the steel–paste interface constitutes a reliable and useful tool for quantitative characterisation of the interface microstructure and thereby provides better insight into the electrochemical processes during corrosion of the steel reinforcement in concrete.     

De-alloying of 316 stainless steel in the presence of a mixture of metal-oxidizing bacteria

The corrosion behavior of 316 stainless steel was investigated in the presence of a mixture of metal-oxidizing bacteria isolated from marine environments. Ennoblement of stainless steel in the presence of these bacteria can lead to localized corrosion that confirmed by ESEM images. The corrosion rate of the metals also increased. EDS results showed that these bacteria caused the segregation of Si elements in the metal structure and promoted the depletion of Cr and Fe contents in the corrosion products. Thus, the de-alloying of stainless steel can occur in the presence of metal-oxidizing bacteria.     

On the penetration of corrosion products from reinforcing steel into concrete due to chloride-induced corrosion

Reinforced concretes were corroded to varying degrees by exposing to cyclic NaCl spray and 40 °C drying. The amount of corrosion products and induced damage were measured using image analysis. We found that corrosion products can accumulate at steel-concrete interface as well as penetrate cement paste and deposit within hydration products, relicts of reacted slag, and air voids. As corrosion increases, the products tend to accumulate at the steel-concrete interface, while the amount penetrating cement paste remains relatively constant. Only a small amount of corrosion is needed to induce visible cover cracking. Implications on modelling time to cover cracking are discussed.     

In situ synchrotron X-ray micro-tomography study of pitting corrosion in stainless steel

Pitting corrosion of stainless steel has been investigated with high-resolution in situ X-ray microtomography. The growth of pits at the tip of stainless steel pins has been observed with 3D microtomography under different conditions of applied current and cell potential. The results demonstrate how pits evolve in stainless steel, forming a characteristic “lacy” cover of perforated metal. In addition, it is shown how the shape of pits becomes modified by MnS inclusions.     

Study on corrosion resistance of palladium films on 316L stainless steel by electroplating and electroless plating

Palladium films with good adhesive strength were deposited on 316L stainless steel by electroless plating and electroplating. Scanning electronic microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, weight loss tests and electrochemical methods were used to study the properties of the films. The electroless plated palladium film mainly consisted of palladium, phosphorus and nitrogen, and the electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples prepared by both methods showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solution, the corrosion rates of the palladium plated 316L stainless steel samples were four orders of magnitude lower than that of the original 316L stainless steel samples. In the solution with 0.01 M NaCl, the palladium plated samples also showed better corrosion resistance. In comparison, the electroplated samples showed slightly better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former.     

Study of localized corrosion of 304 stainless steel under chloride solution droplets using the wire beam electrode

Localized corrosion of 304 stainless steel under droplets of 1 M sodium chloride solution was investigated by the wire beam electrode (WBE) method. It was found that the current distributions were heterogeneous with isolated anodic current peaks mostly located near the edge of the droplet. During the corrosion process, the stainless steel WBE exhibited the stochastic characteristics with the disappearance of some anodic sites. In addition, stainless steel suffered more serious localized corrosion with the increase of the droplet size. The increase of the cathodic area and the three-phase boundary (TPB) length was believed to be the reason.     

Effect of hydrochloric acid on pickling of hot-rolled 304 stainless steel in iron chloride-based electrolytes

Weight loss, corrosion potential and EIS measurements were performed to understand the role of HCl in the pickling of oxidised 304 stainless steel in iron chloride-based electrolytes. The surface finish was analysed with SEM-EDX. The oxidised 304 stainless steel is active on immersion, resulting in a low corrosion potential and a high weight loss. After certain duration the stainless steel either remains active or becomes passive depending on HCl content. At low HCl contents, an ongoing active-to-passive transition results in localised corrosion at pits, grain boundaries and honeycombed recesses. The corrosion potential becomes high and the weight loss is suppressed. The weight loss decreases in the initial stage and rises on extended pickling with adding HCl. Because of anodic brightening, the 304 stainless steel is always active as HCl is concentrated. In contrast with the material that is passivated, the charge transfer resistance is considerably low and the double layer capacitance is large during that brightening.     

Depassivation-repassivation behavior of type-312L stainless steel in NaCl solution investigated by the micro-indentation

Repassivation behavior of type-312L stainless steel containing 6% of molybdenum was examined in NaCl solution using in situ micro-indentation technique, together with type-304 and 316L stainless steels. High stability of the passive film formed on the type-312L stainless steel was also examined by depth profiling analysis of passive films using glow discharge optical emission spectroscopy (GDOES). In 0.9 mol dm⁻³ NaCl solution at 296 K the type-304 and 316L stainless steels are passive only up to 0.3 V (SHE), above which pitting corrosion occurs. In contrast, no pitting corrosion occurs on type-312L stainless steel. Despite the significant difference of the pitting corrosion resistance, the repassivation kinetics of the three stainless steels, examined by micro-indentation at 0.3 V (SHE), is similar. The presence of molybdenum in the stainless steel does not influence the repassivation kinetics. The charge required to repassivate the ruptured type-312L stainless steel surface increases approximately linearly with the potential, even though the passivity-maintaining current increased markedly at potentials close to the transpassive region. Repassivation occurs without accompanying significant dissolution of steel, regardless of the stability of passive state. Depth profiling analyses of the passive films on the type-312L stainless steels formed at several potentials revealed that molybdenum species enrich in the outer layer of the passive film, below which chromium-enriched layer is present. The permeation of chloride ions may be impeded by the outer layer containing molybdate, enhancing the resistance against the localized corrosion of the type-312L stainless steel.     

On the corrosion mechanism of AISI 204Cu stainless steel in chlorinated alkaline media

Electrochemical techniques (CV, SECM, CPT) and surface analysis techniques (EDX, SEM) have been employed to assess the corrosion behaviour of the AISI 204Cu stainless steel. The behaviour of this steel has been compared with that of AISI 304 and AISI 434 stainless steels in chlorinated alkaline media. All samples performed well at room temperature under potentiodynamic polarisation up to a chloride to hydroxyl ratio of 10. At this ratio the AISI 204Cu and the AISI 434 steels presented pitting potential at +0.47 V vs. SCE and +0.31 V vs. SCE, respectively. Moreover, the critical pitting temperature was higher for the AISI 204Cu steel than for the AISI 434 steel, respectively 58 °C and 28 °C.In terms of corrosion performance of the AISI 204Cu stainless steel can be classified better than the AISI 434 steel and worse than the AISI 304 steel.Local electrochemical and chemical examinations allowed evidencing the local activity of some pits over long period, and to conclude that the improved corrosion performance of the low nickel alloy AISI 204Cu stainless steel should be ascribed to copper cementation at active corrosion sites.     

Corrosion properties of plasma nitrided AISI 410 martensitic stainless steel in 3.5% NaCl and 1% HCl aqueous solutions

Samples of an AISI 410 martensitic stainless steel were plasma nitrided at a temperature of 420 °C, 460 °C or 500 °C for 20 h. The composition, microstructure and hardness of the nitrided samples were characterised using a variety of analytical techniques. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarisation tests in 3.5% NaCl solution and immersion tests in 1% HCl acidic water solution. The results showed that plasma nitriding produced a relatively thick nitrided case consisting of a compound layer and a nitrogen diffusion layer on the 410 stainless steel surface. Plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the martensitic stainless steel. In the immersion test, nitrided samples showed lower weight loss and lower corrosion rate than untreated one. In the electrochemical corrosion tests, the nitrided samples showed higher corrosion potentials, higher pitting potentials and greatly reduced current densities. The improved corrosion resistance was believed to be related to the iron nitride compound layer formed on the martensitic stainless steel surface during plasma nitriding, which protected the underlying metal from corrosive attack under the testing conditions.     

Electrochemical corrosion behavior of a novel antibacterial stainless steel

A novel antibacterial stainless steel (ASS) with martenstic microstructure has been recently developed, by controlled copper ion implantation, as a new functional material having broad-spectrum antibacterial properties. The electrochemical corrosion behavior of the ASS in 0.05 mol/L NaCl was assessed using linear polarization and electrochemical impedance spectroscopy (EIS) and compared with that of a conventional stainless steel (SS) without copper ion implantation. The ASS exhibited higher corrosion susceptibility in the chloride medium; with a more negative (active) corrosion potential, higher anodic current density and lower charge transfer and polarization resistance. This has been attributed to the occurrence of copper-catalyzed interfacial reactions. A functional tool, 3-D presentation of EIS data, has been employed in analyzing the electrochemical corrosion processes as well as probing complex interfacial phenomena.     

Effect of ZnO on the corrosion of zinc,Q235 carbon steel and 304 stainless steel under white light illumination

This paper discusses the effects of the coupling of ZnO–Zn, Q235 carbon steel, and 304 stainless steel on their corrosion in 3.5% NaCl under white light illumination. In the presence of Na₂S, ZnO provided photoelectrochemical cathodic protection for the coupled metals. In the absence of Na₂S, ZnO could only provide photoelectrochemical cathodic protection for 304 stainless steel. Under white light illumination, the open circuit potential of ZnO photoelectrode shifted in the negative direction. ZnO provided photoelectrochemical cathodic protection to the metals only when the open circuit potential of ZnO photoelectrode was more negative than that of the metals.     

An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater

The effects of applied torque on corrosion behaviour of 316L stainless steel with crevices were investigated using the cyclic potentiodynamic polarization method. Three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) were tested in artificial seawater at 50 °C. Corroded surface morphology was also investigated using scanning electron microscopy. Results indicate similar trends in crevice corrosion susceptibility with increasing applied torque. Among the three crevices, the 316L stainless steel specimen, coupled to the 316L stainless steel crevice former, is the most susceptible to crevice corrosion.     

Electrochemical noise from oxygen reduction on stainless steel surfaces

Oxygen reduction occurring at the passive layer is probably the most important cathodic reaction involved in corrosion processes on stainless steel. Furthermore, the influence of the surface state on the oxygen reduction reaction is a key point for the understanding of the mechanism of localized corrosion on stainless steel. In this study, electrochemical noise measurements under cathodic polarization were carried out to obtain new information about this influence. It has been confirmed that the surface state of stainless steel plays a very important role in the kinetic of this cathodic reaction. Oxygen reduction kinetics was significantly reduced on passivated surfaces and improved on pre-reduced and ground surfaces. In addition, electrochemical current noise measurements allowed to differentiate between the electrochemical activity produced by the oxygen reduction reaction and that due to the reduction of the passive layer, in direct dependence on the characteristics of the different surface states investigated.     

Influence of outer rust layers on corrosion of carbon steel and weathering steel during wet–dry cycles

The influence of the rust layers of carbon steel and weathering steel on the corrosion were investigated. It was found that corrosion of carbon steel slows down when its outer rust layer is removed. This phenomenon might be attributed to the shortening of the wetting time in wet–dry cycles when the outer rust layers are removed. What is more, growth time of the corrosion products is shortened as well, which results in the formation of the fine corrosion products. However, the behavior of corrosion of weathering steel is not obviously influenced by the outer rust layer and the wetting time.     

Electrodeposition of polypyrrole on 316L stainless steel for corrosion prevention

The electrosynthesis of polypyrrole films onto 316L stainless steel from near neutral and alkaline solutions containing molybdate and nitrate is reported. The corrosion behavior of the coated electrodes was investigated in NaCl solutions by electrochemical techniques and scanning electron microscopy. The polymer formed potentiostatically in a solution of pH 12 is the most efficient in terms of adhesion and corrosion protection. The coating significantly reduces the pitting corrosion of the substrate. The results are interpreted in terms of the nature of dopants, the good electroactivity of the polymer formed in alkaline solution and the passivating properties of the oxide layer.     

Corrosion performance of steel coated with co-polyamides and polyaniline

The corrosion performance of steel coated with co-polyamide polymers and polyaniline in neutral salt spray (NSS) and accelerated corrosion testing is presented. A coating of the nylon polymer on steel is not sufficient to prevent corrosion in a chloride medium, the underlying steel showing signs of corrosion after only three days in the NSS environment. Open-circuit potential measurements indicate the steel is corroding forming soluble ferrous and ferric oxy-complexes. Polyaniline by itself is also insufficient in inhibiting corrosion on steel. A synergy exists between an under-layer of polyaniline and a top layer of the nylon coating in minimizing corrosion.     

Chloride threshold level for corrosion of steel in concrete

The steel rebar inside reinforce concrete structures is susceptible to corrosion when permeation of chloride from deicing salts or seawater results in the chloride content at the surface of the steel exceeding a chloride threshold level (CTL). The CTL is an important influence on the service life of concrete structures exposed to chloride environments. The present study discusses the state of art on the CTL for steel corrosion in concrete, concerning its measurement, representation, influencing factors and methods to enhance the CTL. As the CTL values reported in the majority of previous studies were varied with experimental conditions, corrosion initiation assessment method, the way in which the CTL was represented, direct comparison between the results from different sets and evaluation was subjected to the difficulty. As a result, total chloride by weight of cement or the ratio of [Cl⁻]:[H⁺] is the best presentation of CTL in that these include the aggressiveness of chlorides (i.e. free and bound chlorides) and inhibitive nature of cement matrix. The key factor on CTL was found to be a physical condition of the steel-concrete interface, in terms of entrapped air void content, which is more dominant in CTL rather than chloride binding, buffering capacity of cement matrix or binders. The measures to raise the CTL values using corrosion inhibitor, coating of steel, and electrochemical treatment are also studied.     

Pitting corrosion of Type 430 stainless steel under chloride solution droplet

To clarify the critical relative humidity (RH) to initiate pitting corrosion and the rusting mechanism in a marine atmospheric environment, pitting corrosion of Type 430 stainless steels under drops of MgCl₂ solutions were investigated. A pitting corrosion test was performed at different relative humidities under droplets with various diameters and thicknesses. The probability of pitting decreased as the diameter and thickness decreased. Pitting progressed only when the chloride concentration exceeded 4 M, which is the equilibrium concentration at 80% RH. Accordingly, pitting of Type 430 could be initiated when the RH was less than 80%. Additionally, a pitting corrosion mechanism of Type 430 stainless steel under droplets containing chloride ions is proposed.     

Corrosion properties of active screen plasma nitrided 316 austenitic stainless steel

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO₃ + 25%H₂O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel.