Corrosion inhibitors for the rehabilitation of reinforced concrete

Four corrosion inhibitors namely sodium nitrite, sodium monofluorophosphate, ethanolamine and an alkanolamine-based mixture were studied by immersing mild steel bars for 42 days in model electrolytes of varied pH and chloride concentration which were intended to simulate the pore solution phase present within carbonated and/or chloride-contaminated concrete. Site trials were carried out on sodium monofluorophosphate and the alkanolamine-based inhibitor to study their depth of penetration into concrete. The influence of various carbonating atmospheres on the pore solution chemistry and microstructure of hydrated cement paste was investigated. Physical realkalisation of carbonated cement paste and a calcium nitrite-based corrosion rehabilitation system for chloride-contaminated cement paste were investigated by monitoring ionic transport within the pore solution phase of laboratory specimens. The main findings were as follows: 1,Sodium nitrite, sodium monofluorophosphate, ethanolamine and the alkanolamine-based mixture all behaved as passivating anodic inhibitors of steel corrosion in air-saturated aqueous solutions of varied pH and chloride concentration. 2,Sodium monofluorophosphate failed to penetrate significantly into partially carbonated site concrete when applied as recommended by the supplier. Phosphate and fluoride penetrated 5mm into partially carbonated site concrete treated with sodium monofluorophosphate. 3,The ethanolamine component of the alkanolamine-based inhibitor was found to have penetrated significant depths into partially carbonated site concrete. 4,Carbonating hydrated cement paste over saturated solutions of sodium nitrite resulted in significant concentrations of nitrite in the pore solution of the carbonated paste. Saturated solutions of sodium chloride, ammonium nitrate, magnesium nitrate and sodium dichromate were investigated and identified as alternatives for controlling the relative humidity of the carbonating environment. 5,Hardened carbonated cement paste can by physically realkalised to a limited extent due to the diffusion of hydroxyl ions under saturated conditions. A substantial proportion of the hydroxyl ions that diffused into the carbonated cement paste however, became bound into the cement matrix. Hydroxyl ion concentrations remained below 5mmol/l within the pore solution of the realkalised cement paste. 6, Nitrite ions penetrated significant distances by diffusion within the pore solution of saturated uncarbonated hydrated cement paste.

Corrosion and passivation of steel in concrete

A study of several chemical and electrochemical factors which affect the behaviour of embedded steel in cement pastes and concrete has been made. The effects of internal and external sources of chloride ions on the pore solution chemistry of Portland cement pastes, with and without additions of anodic corrosion inhibitors, have been studied using a pore solution expression device which has enabled samples of pore solution to be expressed from hardened cement pastes and analysed for various ionic species. Samples of pure alite and tricalcium aluminate have been prepared and characterised with respect to morphology, free lime content and fineness. Kinetics of diffusion of chloride ions in hardened pastes of alite and alite blended with tricalcium aluminate have been investigated and an activation energy obtained for the diffusion process in alite. The pore structures of the hardened pastes and the chloride ion binding capacity of alite have also been determined. Concrete cylinders containing embedded steel with four different surface conditions were exposed to various environments. The electrochemical behaviour of the steel was monitored during the period of exposure by means of rest potential measurements and the steel corrosion products analysed before and after being embedded. An examination was made of the nature of the interfacial zones produced between the embedded steel and cement. Rest potential measurements were monitored for steel embedded in alite paste in the presence of chloride ions and cement paste containing various levels of inhibitors in combination with chloride ions. In the latter case the results were supported by polarisation resistance determinations.

Corrosion mechanisms of duplex stainless steels in the petrochemical industry

The recent search for new sources of hydrocarbons has led to production from very severe environments which can contain considerable amounts of carbon dioxide, hydrogen sulphide, and chloride ions, combined with temperatures which can exceed 100°C. Oil and gas production from such wells requires highly corrosion-resistant materials. The traditional solution of using carbon steel with additional protection is generally inadequate in these very-aggressive environments. Duplex stainless steels (DSS) are attractive candidates because of their high strength, good general corrosion resistance, excellent resistance to chloride-induced stress corrosion cracking, and good weldability. Although duplex stainless steels have a very good reputation in both subsea and topsides pipework, it is recognized that the tolerance of these materials to variations in microstructure and chemical composition are still not fully understood. The object of this paper is to review the corrosion behaviour of duplex stainless steels in the petrochemical industry, with particular emphasis on microstructures and the effect of changes in chemical composition.

Toughness variations during the tempering of a plain carbon, martensitic steel

The aim of this paper is to present results of toughness measurements and fractographic observations made on a quenched and tempered 0. 6%, carbon steel and to use these as a basis for a discussion of the factors affecting "623K (350 °C) embrittlement" (one-step temper embrittlement).

Characterization of oxide layers grown on D9 austenitic stainless steel in lead bismuth eutectic

Lead bismuth eutectic (LBE) is a possible coolant for fast reactors and targets in spallation neutron sources. Its low melting point, high evaporation point, good thermal conductivity, low reactivity, and good neutron yield make it a safe and high performance coolant in radiation environments. The disadvantage is that it is a corrosive medium for most steels and container materials. This study was performed to evaluate the corrosion behavior of the austenitic stainless steel D9 in oxygen controlled LBE. In order to predict the corrosion behavior of steel in this environment detailed analyses have to be performed on the oxide layers formed on these materials and various other relevant materials upon exposure to LBE. In this study the corrosion/oxidation of D9 stainless steel in LBE was investigated in great detail. The oxide layers formed were characterized using atomic force microscopy, magnetic force microscopy, nanoindentation, and scanning electron microscopy with wavelength-dispersive spectroscopy (WDS) to understand the corrosion and oxidation mechanisms of D9 stainless steel in contact with the LBE. What was previously believed to be a simple double oxide layer was identified here to consist of at least 4 different oxide layers. It was found that the inner most oxide layer takes over the grain structure of what used to be the bulk steel material while the outer oxide layer consists of freshly grown oxides with a columnar structure. These results lead to a descriptive model of how these oxide layers grow on this steel under the harsh environments encountered in these applications.

The Mechanics of the drawing of polygonal sections from round at elevated temperature

Economic factors such as the rise in cost of raw materials, labour and power, are compelling manufacturers of cold-drawn polygonal sections, to seek new production routes which will enable the expansion in the varieties of metals used and the inclusion of difficult-to-draw materials. One such method generating considerable industrial interest is the drawing of polygonal sections from round at elevated temperature. The technique of drawing mild steel, medium carbon steel and boron steel wire into octagonal, hexagonal and square sections from round at up to 850 deg C and 50% reduction of area in one pass has been established. The main objective was to provide a basic understanding of the process, with particular emphasis being placed on modelling using both experimental and theoretical considerations. Elevated temperature stress-strain data was obtained using a modified torsion testing machine. Data were used in the upper bound solution derived and solved numerically to predict drawing stress strain, strain-rate, temperature and flow stress distribution in the deforming zone for a range of variables. The success of this warm working process will, of course, depend on the use of a satisfactory elevated temperature lubricant, an efficient cooling system, a suitable tool material having good wear and thermal shock resistance and an efficient die profile design which incorporates the principle of least work. The merits and demerits of die materials such as tungsten carbide, chromium carbide, Syalon and Stellite are discussed, principally from the standpoint of minimising drawing force and die wear. Generally, the experimental and theoretical results were in good agreement, the drawing stress could be predicted within close limits and the process proved to be technically feasible. Finite element analysis has been carried out on the various die geometries and die materials, to gain a greater understanding of the behaviour of these dies under the process of elevated temperature drawing, and to establish the temperature distribution and thermal distortion in the deforming zone, thus establishing the optimum die design and die material for the process. It is now possible to predict, for the materials already tested, (i) the optimum drawing temperature range, (ii) the maximum possible reduction of area per pass, (iii) the optimum drawing die profiles and die materials, (iv) the most efficient lubricant in terms of reducing the drawing force and die wear.

The behaviour of polymer quenchants

The internationally accepted Wolfson Heat Treatment Centre Engineering Group test was used to evaluate the cooling characteristics of the most popular commercial polymer quenchants: polyalkylene glycols, polyvinylpyrrolidones and polyacrylates. Prototype solutions containing poly(ethyloxazoline) were also examined. Each class of polymer was capable of providing a wide range of cooling rates depending on the product formulation, concentration, temperature, agitation, ageing and contamination. Cooling rates for synthetic quenchants were generally intermediate between those of water and oil. Control techniques, drag-out losses and response to quenching in terms of hardness and residual stress for a plain carbon steel, were also considered. A laboratory scale method for providing a controllable level of forced convection was developed. Test reproducibility was improved by positioning the preheated Wolfson probe 25mm above the geometric centre of a 25mm diameter orifice through which the quenchant was pumped at a velocity of 0.5m/s. On examination, all polymer quenchants were found to operate by the same fundamental mechanism associated with their viscosity and ability to form an insulating polymer-rich-film. The nature of this film, which formed at the vapour/liquid interface during boiling, was dependent on the polymer's solubility characteristics. High molecular weight polymers and high concentration solutions produced thicker, more stable insulating films. Agitation produced thinner more uniform films. Higher molecular weight polymers were more susceptible to degradation, and increased cooling rates, with usage. Polyvinylpyrrolidones can be cross-linked resulting in erratic performance, whilst the anionic character of polyacrylates can lead to control problems. Volatile contaminants tend to decrease the rate of cooling and salts to increase it. Drag-out increases upon raising the molecular weight of the polymer and its solution viscosity. Kinematic viscosity measurements are more effective than refractometer readings for concentration control, although a quench test is the most satisfactory process control method.

Nitrogen implantation of tool steels and engineering coatings

Ion implantation modifies the surface composition and properties of materials by bombardment with high energy ions. The low temperature of the process ensures the avoidance of distortion and degradation of the surface or bulk mechanical properties of components. In the present work nitrogen ion implantation at 90 keV and doses above 1017 ions/cm2 has been carried out on AISI M2, D2 and 420 steels and engineering coatings such as hard chromium, electroless Ni-P and a brush plated Co-W alloy. Evaluation of wear and frictional properties of these materials was performed with a lubricated Falex wear test at high loads up to 900 N and a dry pin-on-disc apparatus at loads up to 40 N. It was found that nitrogen implantation reduced the wear of AISI 420 stainless steel by a factor of 2.5 under high load lubricated conditions and by a factor of 5.5 in low load dry testing. Lower but significant reductions in wear were achieved for AISI M2 and D2 steels. Wear resistance of coating materials was improved by up to 4 times in lubricated wear of hard Cr coatings implanted at the optimum dose but lower improvements were obtained for the Co-W alloy coating. However, hardened electroless Ni-P coatings showed no enhancement in wear properties. The benefits obtained in wear behaviour for the above materials were generally accompanied by a significant decrease in the running-in friction. Nitrogen implantation hardened the surface of steels and Cr and Co-W coatings. An ultra-microhardness technique showed that the true hardness of implanted layers was greater than the values obtained by conventional micro-hardness methods, which often result in penetration below the implanted depth. Scanning electron microscopy revealed that implantation reduced the ploughing effect during wear and a change in wear mechanism from an abrasive-adhesive type to a mild oxidative mode was evident. Retention of nitrogen after implantation was studied by Nuclear Reaction Analysis and Auger Electron Spectroscopy. It was shown that maximum nitrogen retention occurs in hard Cr coatings and AISI 420 stainless steel, which explains the improvements obtained in wear resistance and hardness. X-ray photoelectron spectroscopy on these materials revealed that nitrogen is almost entirely bound to Cr, forming chromium nitrides. It was concluded that nitrogen implantation at 90 keV and doses above 3x1017 ions/cm2 produced the most significant improvements in mechanical properties in materials containing nitride formers by precipitation strengthening, improving the load bearing capacity of the surface and changing the wear mechanism from adhesive-abrasive to oxidative.

Metakaolin as a cement extender

The effect of 10% and 20% replacement metakaolin on a number of aspects of hydration chemistry and service performance of ordinary Portland cement pastes has been investigated. The analysis of expressed pore solutions has revealed that metakaolin-blended specimen pastes possess enhanced chloride binding capacities and reduced pore solution pH values when compared with their unblended counterparts. The implications of the observed changes in pore solution chemistry with respect to chloride induced reinforcement corrosion and the reduction in expansion associated with the alkali aggregate reaction are discussed. Differential thermal analysis, mercury intrusion porosimetry, and nuclear magnetic resonance spectroscopy have been employed in the analysis of the solid phase. It is suggested that hydrated gehlenite (a product of pozzolanic reaction) is operative in the removal and solid state binding of chloride ions from the pore solution of metakaolin-blended pastes. Diffusion coefficients obtained in a non-steady state chloride ion diffusion investigation have indicated that cement pastes containing 10% and 20% replacement metakaolin exhibit superior resistance to the penetration of chloride ions in comparison with those of plain OPC of the same water:cement ratio. The chloride induced corrosion behaviour of cement paste samples, of water:cement ratio 0.4, containing 0% , 10%, and 20% replacement metakaolin, has been monitored using the linear polarization technique. No significant corrosion of embedded mild steel was observed over a 200 day period.

The influence of Arabian Gulf environment on mechanisms of reinforcement corrosion

The reduction in the useful-service life of reinforced concrete construction in the Arabian Gulf is attributed to reinforcement corrosion. While this phenomenon is primarily related to chloride ions, the concomitant pressure of sulfate salts may accelerate the deterioration process. Another factor which might influence reinforcement corrosion is the elevated ambient temperature. While few studies have been conducted to evaluate the individual effect of sulfate contamination and temperature on chloride binding and reinforcement corrosion, the synergistic effect of these factors on concrete durability, viz.-a-viz., reinforcement corrosion, needs to be evaluated. Further, the environmental conditions of the Arabian Gulf are also conducive for accelerated carbonation. However, no data are available on the concomitant effect of chloride-sulfate contamination and elevated temperature on the carbonation behaviour of plain and blended cements.This study was conducted to evaluate the conjoint effect of chloride-sulfate contamination and temperature on the pore solution chemistry and reinforcement corrosion. The effect of chloride-sulfate contamination and elevated temperature on carbonation in plain and blended cements was also investigated. Pore solution extraction and analysis, X-ray diffraction, differential thermal analysis, scanning electron microscopy, DC linear polarization resistance and AC impedance spectroscopy techniques were utilized to study the effect of experimental parameters on chloride binding, reinforcement corrosion and carbonation.The results indicated that the concomitant presence of chloride and sulfate salts and temperature significantly influences the durability performance of concrete by: (i) decreasing the chloride binding, (ii) increasing reinforcement corrosion, and (iii) accelerating the carbonation process. To avoid such deterioration, it is advisable to minimize both chloride and sulfate contamination contributed by the mixture ingredients. Due to the known harmful role of sulfate ions in decreasing the chloride binding and increasing reinforcement corrosion, limits on allowable sulfate contamination in concrete should also be established.

CO2 absorption into aqueous amine blended solutions containing monoethanolamine (MEA), N,N-dimethylethanolamine (DMEA), N,N-diethylethanolamine (DEEA) and 2-amino-2-methyl-1-propanol (AMP) for post-combustion capture processes

Presently monoethanolamine (MEA) remains the industrial standard solvent for CO2 capture processes. Operating issues relating to corrosion and degradation of MEA at high temperatures and concentrations, and in the presence of oxygen, in a traditional PCC process, have introduced the requisite for higher quality and costly stainless steels in the construction of capture equipment and the use of oxygen scavengers and corrosion inhibitors. While capture processes employing MEA have improved significantly in recent times there is a continued attraction towards alternative solvents systems which offer even more improvements. This movement includes aqueous amine blends which are gaining momentum as new generation solvents for CO2 capture processes. Given the exhaustive array of amines available to date endless opportunities exist to tune and tailor a solvent to deliver specific performance and physical properties in line with a desired capture process. The current work is focussed on the rationalisation of CO2 absorption behaviour in a series of aqueous amine blends incorporating monoethanolamine, N,N-dimethylethanolamine (DMEA), N,N-diethylethanolamine (DEEA) and 2-amino-2-methyl-1-propanol (AMP) as solvent components. Mass transfer/kinetic measurements have been performed using a wetted wall column (WWC) contactor at 40°C for a series of blends in which the blend properties including amine concentration, blend ratio, and CO2 loadings from 0.0-0.4 (moles CO2/total moles amine) were systematically varied and assessed. Equilibrium CO2 solubility in each of the blends has been estimated using a software tool developed in Matlab for the prediction of vapour liquid equilibrium using a combination of the known chemical equilibrium reactions and constants for the individual amine components which have been combined into a blend.From the CO2 mass transfer data the largest absorption rates were observed in blends containing 3M MEA/3M Am2 while the selection of the Am2 component had only a marginal impact on mass transfer rates. Overall, CO2 mass transfer in the fastest blends containing 3M MEA/3M Am2 was found to be only slightly lower than a 5M MEA solution at similar temperatures and CO2 loadings. In terms of equilibrium behaviour a slight decrease in the absorption capacity (moles CO2/mole amine) with increasing Am2 concentration in the blends with MEA was observed while cyclic capacity followed the opposite trend. Significant increases in cyclic capacity (26-111%) were observed in all blends when compared to MEA solutions at similar temperatures and total amine concentrations. In view of the reasonable compromise between CO2 absorption rate and capacity a blend containing 3M MEA and 3M AMP as blend components would represent a reasonable alternative in replacement of 5M MEA as a standalone solvent.