Investigation of pitting potential of carbon steel using experimental design method

Abstract

The dependence of the pitting potential E_p of 1018 carbon steel on chloride concentration, pH and the temperature of the solution was studied by the potentiodynamic method in accordance with a statistical experimental design. The parameters of the empirical pitting potential model determined on the basis of Box-W ilson ex perimental design method were evaluated using the experimental data. Comparison of the predicted values from the model with the observed values showed that the model is a good fit. From the model equation the most noble pitting potential value of -225 mV (SCE) was obtained when the Cl^- ion concentration, temperature and pH of the solution were 205 ppm, 25°C, and 6·4, respectively. The Box-Wilson experimental design technique was proved to be applicable in modelling the pitting potential of carbon steel.     

Effect of industrial surface finishes on heat treatment and surface engineering of steel

Abstract

The surface state of industrial parts is complex and effective heat treatment or surface engineering of such parts requires a precise knowledge of how the steel was produced and manufactured. During manufacturing a mechanical part will be subject to the actions of various tools, mechanical constraints and diverse: solids, liquids or gases. Current highly automated processes have introduced new materials and chemicals which change the characteristics of the surface and may generate defects or even scrap. A series of case studies is presented illustrating the problems arising from industrial processes and their solutions.     

Towards industrialisation of microarc oxidation (MAO) for ceramic conversion surface treatment of magnesium and aluminium alloys

Abstract

Microarc oxidation (MAO), as a novel surface treatment, has demonstrated significant achievements both in academic research and in industrial application. Here, the electrical energy consumption and the role of the electrolyte during the MAO process are discussed. The corrosion resistance of MAO coatings on a magnesium alloy has been evaluated by salt spray test and electrochemical analysis. The wear resistance of MAO coating on an aluminium alloy has been tested by an abrasion technique. Finally, application prospects are discussed.     

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Abstract

Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0·045–0·055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h^?1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h^?1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO₂ laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO₂ laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h^?1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO₂ laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO₂ laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.     

Effects of surface conditions on resistance spot welding of Mg alloy AZ31

Abstract

In this work, resistance spot welding of Mg alloy AZ31 sheets was investigated in as received and acid cleaned surface conditions. As received sheets had higher contact resistance which required lower current thresholds for weld initiation and for four root t nugget size (where t is sheet thickness). However, it also led to both serious expulsion and internal defects. The fracture mode of welds in as received sheets was interfacial failure while that of the acid cleaned specimens shifted from interfacial to nugget pullout and exhibited better strength. The acid cleaned sheets also produced less damage on electrode tip faces.     

Nitinol surface finishing by magnetoelectropolishing

Abstract

A new advanced technology is proposed to obtain bare metal surfaces for peripheral stents, endodontic files and another medical devices. The effect of an externally applied magnetic field during electropolishing (magnetoelectropolishing) of Nitinol on its surface properties was investigated. The results are compared with standard electropolished Nitinol in the same electrolyte under the same process conditions. To evaluate surface properties, the following techniques were used: SEM and EDX, AFM, contact angle measurement, Auger electron spectroscopy, electrochemical impedance spectroscopy and fatigue resistance testing. All of the above analyses showed significant differences between magnetoelectropolished and conventional electropolished Nitinol surfaces. The two most prominent differences, which are likely to have the most profound implications on increased use of this intermetallic material in medical application are improved wettability and fatigue resistance.     

Microbial Corrosion Hazard in Fuel Storage Tanks in the Presence of Corrosion Inhibitors

Abstract

Tests were carried out in five light fuel oil storage tanks placed outdoors in the vicinity of Stockholm, Sweden, to study the effectiveness of some commercially available corrosion inhibitors in the presence of certain micro-organisms. Corrosion in the aqueous phase caused by Desulphovibrio desulphuricans, Thiobacillus ferro-oxidans and Cladosporium resinae was effectively stopped by the addition of a sodium nitrite-sodium tetraborate mixture, but was enhanced by each of three commercial oil-soluble inhibitors. In some cases the corrosion rate exceeded 3 mm/year. The results are of a preliminary nature, but give cause for alarm and suggest the need for further investigation.     

Use of time-modulated AC magnetic fields for melt flow control during unidirectional solidification

Abstract

A new electromagnetic stirring approach using a combination of rotating (RMF) and travelling magnetic fields (TMF) is proposed, where both fields are applied subsequently in form of rectangular pulses. The strategy to utilise time-modulated RMF and TMF is aimed at overcoming the known deficiencies of conventional stirring, in particular flow-induced macrosegregation. This paper considers the directional solidification of Al–Si alloys from a water cooled copper chill. The results demonstrate that melt agitation using modulated magnetic field offers a considerable potential for a well aimed modification of casting properties by an effective control of the flow field, but, this goal requires a well considered optimisation of the magnetic field parameters.     

Ecologically green conversion coating for zinc–cobalt alloy

Abstract

A phytic acid based conversion coating has been designed for Zn–Co alloys. The morphology of this coating was studied by SEM and showed that immersion of coatings for 15 min is smooth and compact. The composition of conversion coatings was analysed by X-ray photoelectron spectroscopy. The analysis data showed that the coating is composed of Zn, C, P, N and O. The presence of Co in the coating was not established. Corrosion protection provided by this coating was evaluated by electrochemical measurements (polarisation curves, electrochemical impedance spectroscopy) in a 0·5M NaCl solution. Electrochemical corrosion measurement showed that the corrosion resistance of the Zn–Co alloy has been improved by the conversion treatment with phytic acid.     

Influence of SiCp content and matrix composition on corrosion resistance in cast aluminium matrix composites in salt fog

Abstract

A study has been carried out into the influence of the proportion of SiCp and the matrix composition of four aluminium metal matrix composites (A360/SiC/10p, A360/SiC/20p, A380/SiC/10p and A380/SiC/20p) on their salt fog corrosion behaviour. The matrix of the A360/SiC/xxp composites is virtually free of Ni and Cu while the A380/SiC/xxp matrix contains 1.39 - 1.44 wt-%Ni and 3.13 - 3.45 wt-%Cu. The kinetics of the corrosion process were studied using gravimetric tests. The nature of corrosion products was analysed by SEM and low angle XRD before and after accelerated testing to determine the influence of microstructural changes on corrosion behaviour during exposure to the corrosive environment. The extent of the corrosion damage to the Al/SiC composites depended on the concentration of nucleation sites and the matrix composition. One of these nucleation sites is in the interface region between the matrix and the particles. The corrosion process was influenced more by the concentration of alloy elements in the matrix than by the proportion of SiCp reinforcement.     

Alternative methodology for on site monitoring of corrosion and remediation of reinforced concrete

Abstract

Difficulties associated with the interpretation of site data collected over long periods of time from commonly used corrosion monitoring techniques for steel reinforcement in concrete, such as corrosion potential measurements and linear polarisation resistance, often make it difficult to assess accurately the extent of corrosion. An alternative methodology for the interpretation of data is proposed, based upon a model of the quality of the passive film on the steel surface. This model leads to a representation of the corrosion state by means of the relationship, over a long period of time, between the corrosion potential and the logarithm of the linear polarisation resistance. For the reinforced concrete panels tested in this study, data points representing this relationship closely fitted a family of results. This led to the development of a so called 'monitoring control diagram', MCD, in which for a fixed geometry and fixed experimental conditions, a relationship between the corrosion potential and polarisation resistance of steel exists. The establishment of an MCD may enable the development of a useful monitoring tool.     

Mushy zone coarsening with fluid flow

Abstract

Cylindrical samples of a near-eutectic AlCu30 alloy are annealed applying constant axial temperature gradients to directionally molten samples in an aerogel furnace. During annealing with various times and gradients also a rotating magnetic field (RMF) of 6 mT was applied leading to azimuthal and meridional flows of well known magnitude. The specific surface area of the primary phase was measured on metallographic in section perpendicular to the sample axis with a fixed amount of fraction solid. The specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and compared to isothermal coarsening measurements of Voorhees and co-workers.     

Corrosion Behavior of IN-800 Superalloy in Waste-Incineration Environments: Hot Corrosion by Molten Chlorides

The corrosion resistance of the IN-800superalloy in contact with a molten mixture of (52-48)mol.% PbCl₂-KCl, similar to that found inwaste-incineration plants, has been studied. Thecorrosion kinetics have been analyzed using continuous-currentelectrochemical techniques and electrochemical-impedancespectroscopy (EIS). Studies were performed to determinethe influence of temperature and of the presence of carbon in the salt on the corrosion rate.Scanning electron microscopy (SEM) and electron probemicroanalysis (EPMA) were used as additional techniquesto analyze the corrosion products in order to elucidate the corrosion mechanism.     

Corrosion Behavior of 12CrMoV Steel in Waste Incineration Environments: Hot Corrosion by Molten Chlorides

The corrosion resistance of a 12CrMoV alloy incontact with a molten mixture of (52-48)mol.%PbCl₂-KCl, similar to that found inwaste incineration plants, has been studied. Thecorrosion kinetics have been analyzed using continuous-currentelectrochemical techniques and electrochemical impedancespectroscopy (EIS). Studies were performed to determinethe influence that temperature and the presence of carbon in the salt have on the corrosion rate.Scanning electron microscopy (SEM) and electron-probemicroanalysis (EPMA) were used as additional analyticaltechniques to analyze the corrosion products in order to elucidate the corrosionmechanism.     

Effect of zinc plating of low carbon steel on corrosion resistance in cassava fluid environment

Abstract

This research work investigated the corrosion resistance of zinc plated low carbon steel in cassava fluid (i.e. containing hydrogen cyanide). It simulated the effect of continuous use of the material in a cyanide environment where corrosion products are left in place. Low carbon steel samples were zinc electroplated at voltages between 0˙5 and 0˙9 V for 5 to 20 min. The plated samples were then subjected to a cassava fluid environment for 30 days. The electrode potentials, in mV (SCE), were measured every day. Weight loss was determined at intervals of 5 days for duration of the exposure period. The result showed corrosion attack on the zinc plated steel, the severity increasing with increasing weight of zinc coating on substrate. The result showed that thinly plated low carbon steel did not have any advantage over unplated steel and were quickly stripped of their zinc plating with resultant corrosion of the underlying steel substrate. Heavily zinc plating steel was observed to offer some protection for the steel but not for a long time. The pH of the cassava solution which initially was acidic because of the cyanide content in the cassava was observed to progress to neutrality after 5 days and then became slightly alkaline at the end of the 30 days test (because of corrosion product contamination of the cyanide), contributing to the reduced corrosion rate. Unplated steel was found to be unsuitable for the fabrication of cassava processing machinery without some form of surface treatment, but unfortunately, zinc is not suitable as a protective coating in this environment.     

Citric acid as natural corrosion inhibitor for aluminium protection

Abstract

The inhibition effect of citric acid on the corrosion behaviour of aluminium in 2M NaCl solution (pH 2) was studied with the help of potentiodynamic polarisation, electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) techniques. The experimental results showed that citric acid inhibits the corrosion of aluminium in NaCl solution and the inhibition efficiency depends on its concentration. The protection efficiency increases with citric acid concentration up to a critical value of 1·0 × 10^?5M. At higher concentrations, the inhibition efficiency reduced again with increasing concentration. The mechanism of inhibition was attributed to the adsorption of citric acid onto the metal surface.     

Development of high quality thermal spraying process by shielding control

Abstract

The shielding controlled plasma spraying process was investigated to improve corrosion resistance of metal surfaces. In this process, a shielding nozzle that covered only spraying area was attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet was controlled by nitrogen flow. When the oxygen concentration in the shielding nozzle was maintained at 0·5%, the metal oxide contents in CoNiCrAlY coating and the porosity of the coating reduced to 0·2 and 0·3% at optimal spray particle size respectively. The corrosion potential in an acid solution including chloride ions was almost constant for 1000 h for CoNiCrAlY coating deposited by shielding controlled plasma spraying. On the other hand, those obtained by atmospheric plasma spraying process decreased in the corrosion potential largely after 10 h. Thus, it can be concluded that the shielding controlled plasma spraying process improves the corrosion resistance of the metal.     

Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Influence of heat treatments and chemical composition on SCC susceptibility during repairing procedure of overlaying of Inconel 182 by laser surface melting

Abstract

A practical repairing technique using laser surface melting (LSM) was developed to remove the stress corrosion cracking (SCC) in overlaying of Inconel 182. Influence of microstructure of different heat treatments performed during repairing process on intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility was discussed. The intergranular precipitate was identified as M₂₃ C₆ by TEM. The microstructure with no intergranular precipitate and refiner sub-grain after LSM process shows excellent IGC/IGSCC resistance. The stress relief heat treatment induced severe microstructure of high IGC/IGSCC susceptibility, owing to the semicontinuous intergranular precipitation. The influence of Nb/C ratio on IGC/IGSCC susceptibility of three nickel based superalloys after LSM process was also investigated. For both of the Inconel 182 alloys with different Nb content, the microstructure after LSM process and following sensitisation treatment showed precipitation free grain boundary. The results of corrosion tests also indicated that the material with higher Nb/C ratio showed higher IGC/IGSCC resistance after LSM process and following sensitisation treatment.     

Corrosion resistance of Ni/Al2O3 coatings in NaCl solution

Abstract

This paper presents results of a research on the corrosion resistance of composite Ni/Al₂O₃ coatings electrochemically deposited from Watts baths containing different amounts (0, 20, 100 g dm^?3) of Al₂O₃ particles. Potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 3% solution of NaCl. The coatings with about 6 wt-% of corundum, deposited from a bath with 100 g dm^?3 of a powder, showed the best protective properties. The rate of corrosion of such coatings after 7 days of exposure in the NaCl solution was over two times slower than that of coatings containing 2 wt-% of Al₂O₃ and six times slower than that of a standard nickel coating. Two equivalent circuits: one consisting of two RC circuits and the other one made up of three RC circuits were used for the analysis of the impedance spectra. Regardless of the presence and amount of the Al₂O₃ particles in the nickel coating, during first day of exposure in the NaCl solution a layer of nickel oxides and hydroxides forms on the surface of the coatings increasing their corrosion resistance. In the case of coatings with 6 wt-% of Al₂O₃, the passive layer is least vulnerable to the aggressive action of Cl^? ions.