1‐Methyl‐3‐pyridine‐2‐yl‐thiourea as inhibitor for acid corrosion of stainless steel

The inhibiting effect of 1‐methyl‐3‐pyridine‐2‐yl‐thiourea (MPT) on the corrosion of stainless steel in 0.5 M H₂SO₄ solution was investigated using weight loss and potentiostatic polarization techniques. The percentage inhibition efficiencies and surface coverage degrees increased with increasing additive concentration. Potentiostatic polarization studies revealed that MPT is of the mixed‐type inhibitor in 0.5 M H₂SO₄ solution. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25–55 °C E_a, ΔH_a and possible mechanism have been proposed. Scanning electron microscopy (SEM) was done from the surface of the exposed sample indicating uniform film on the surface.     

High‐temperature corrosion behavior of some post‐plasma‐spraying‐gas‐nitrided metallic coatings on a Fe‐based superalloy

The objective of the present study is to propose a cost‐effective process for modifying commercially available coatings by gas nitriding using commonly available equipment and starting materials. Al–Cr and Ti–Al metallic coatings were deposited on Superfer 800H (Fe‐based superalloy) using a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma‐sprayed‐coated specimens. Characterization and high‐temperature corrosion behavior of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDX, and X‐ray mapping analysis were used for the characterization of the coatings and analysis of the oxide scale. Both the coatings successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation (Type‐I hot corrosion) at 900°C for 50 cycles in air and molten salt (a salt mixture of Na₂SO₄–60%V₂O₅ dissolved in distilled water). Based on the findings of the present study, the coatings under study are recommended for tapplications to super‐heater and reheater tubes of boilers and all those surfaces that face fireside corrosion, such as fluidized beds, industrial waste incinerators, internal combustion engines, gas turbines or steam turbines, to provide protection against degradation in these environments. The cost of the product/process is approximately Rs. 0.62 per mm² in case of Al–Cr coating and Rs. 1.86 per mm² in case of Ti–Al coating.     

Laboratory‐simulated fuel‐ash corrosion of superheater tubes in coal‐fired ultra‐supercritical‐boilers

The corrosion behaviour of a number of Ni‐base alloys of different composition together with a highly‐alloyed austenitic stainless steel has been investigated with and without coating of the samples with alkali‐containing synthetic coal ash. The effects of various parameters have been studied, namely the Cr content (from 20 to 28 mass % Cr), the test temperature (700 and 775°C), the SO₂ content in the gas (1 vol% and 0.25 vol % SO₂) and the alkali sulphate content of the ash (10 and 30 mass%). The results can be explained on the basis of the differences in corrosion mechanisms resulting primarily from the composition of the alloys tested.     

Korrosionsverhalten von Nd‐Fe‐B‐Dauermagneten

Corrosion behaviour of Nd‐Fe‐B permanent magnets The corrosion behaviour of Nd‐Fe‐B permanent magnetic alloys as well as of single phases of these alloys have been investigated in sulphuric acid at room temperature and humid air at 150 °C using mass loss and electrochemical techniques. Scanning electron microscopy and scanning probe microscopy were used to study the surface topography. The electrostatic surface potential was examined by scanning probe microscopy using tapping mode. A correlation between the electrostatic surface potential and the corrosion rate of these alloys was found. The higher the value of the electrostatic surface potential of the intergranular phases the higher is the corrosion sensitivity. The strength of the corrosion attack on the phases of sintered permanent magnetic alloys is as follows: Ferrmagnetic phase < B‐rich phase < Nd‐rich phase. The differences in the chemical composition as well as in the preparation of these magnetic alloys have distinct influence on the corrosion resistance of the magnetic Nd‐Fe‐B alloys.     

Electrochemical evaluation of Ti‐13Nb‐13Zr,Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys for biomedical application by long‐term immersion tests

In this investigation the electrochemical behaviour of the Ti‐13Nb‐13Zr, Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys, for application as implant materials was evaluated in Hanks' solution by electrochemical techniques. The alloys were immersed in this solution for 410 days and periodically they were tested by electrochemical impedance spectroscopy. At the end of this period, polarization curves of the three titanium alloys were obtained. The electrochemical impedance experimental results were interpreted using an equivalent electrical circuit that simulates a duplex structure oxide composed of an inner compact layer, here called barrier layer, and an outer and porous layer. The results indicated that all the alloys present a very high corrosion resistance in the electrolyte used, typical of passive alloys, and that the corrosion resistance is mainly due to the barrier layer. The passive like behaviour was maintained during the whole period of test.     

Early‐stage oxidation behavior of Co‐rich high‐temperature alloys

The long‐term oxidation performance of an alloy is critically linked to the early‐stage oxidation behavior of high‐temperature alloys. This study investigates early‐stage oxidation behavior in terms of oxidation kinetics, scale evolution, and residual stresses developed within a scale of the commercially available cobalt‐rich alloys: HAYNES® 188, 6B, 25, and HR‐160® and a newly developed nitride‐dispersion strengthened NS‐163® alloy (HAYNES®, HR‐160®, NS‐163® are registered trademarks of Haynes International, Inc). Short‐term isothermal oxidation exposures were conducted in flowing air at 982 °C for durations of 1–50 h. Oxidation kinetics was assessed by weight‐change behavior, which showed that 188 alloy exhibited the lowest weight‐gain, while for similar times HR‐160 alloy underwent weight‐loss. SEM/EDS analysis was performed to characterize oxides formed in these alloys, while stresses developed in the oxides of different alloys were measured using synchrotron X‐ray radiation. The results in this paper clearly demonstrated the effects of alloy composition on the scale evolution and the amount of stresses developed in oxides.     

1,3‐Bis‐dibutylaminopropan‐2‐ol as inhibitor for reinforcement steel in chloride‐contaminated simulated concrete pore solution

An organic compound, 1,3‐bis‐dibutylaminopropan‐2‐ol (BDAP) was prepared and tested experimentally as inhibitor for reinforcement steel in simulated concrete pore solution. The electrochemical behavior of carbon steel electrodes immersed in simulated pore solutions with different concentration of BDAP was investigated by electrochemical measurements. Adsorption isotherm was determined based on the inhibition efficiency. The surface composition was evaluated via X‐ray photoelectron spectroscopy (XPS) as well in order to verify the presence and adsorption property of BDAP. The results showed that BDAP could effectively suppress the anodic process of carbon steel corrosion and the inhibition efficiency was improving with the increase of inhibitor concentration. BDAP could adsorb on carbon steel surface according to Langmuir adsorption isotherm. XPS spectrum certified the existence of BDAP on the steel surface.     

Influence of atmosphere on high‐temperature oxidation of Fe‐Cr‐Si model alloy

The aim of this study is to show the influence of the gaseous environment on the ferritic Fe‐Cr‐Si model alloy oxidation during 70 hrs, at 900 and 950°C. Two different atmospheres have been used, air or nitrogen containing 5 vol% hydrogen (N₂–5%H₂). After air oxidation, a nonadherent chromia scale formed. In the N₂–5%H₂ gaseous environment, it clearly appears that silicon segregation near the internal scale‐metal interface is favoured. In this low oxygen‐containing gas, adherent chromia, and silica scales have been formed. Silica subscale associated to an adherent chromia scale obtained in low‐oxygen conditions are a good protection barrier against carburisation.     

Electrochemical and mechanical behaviour of Sn‐2.5Ag‐0.5Cu and Sn‐48Bi‐2Zn solders

In this paper, two tin‐based alloys (Sn‐2.5Ag‐0.5Cu and Sn‐48Bi‐2Zn) are proposed as new lead‐free solders. Alloys have been developed by melting pure elements. Samples have been evaluated in terms of microstructure, corrosion resistance and mechanical features. Corrosion tests have been performed in 3% NaCl solution by polarization curves and electrochemical impedance spectroscopy (EIS). SEM observations and EDS analysis were carried out on samples before and after corrosion tests. Static monotonic tensile tests have been performed on three specimens for each alloy. SEM and EDS analysis revealed the presence of Sn‐Ag and Sn‐Cu intermetallic compounds within the Sn‐Ag‐Cu alloy. As a result of corrosion test, the Sn‐Ag‐Cu alloy showed a better corrosion resistance with respect to Sn‐Bi‐Zn. Both alloys evidenced good mechanical properties higher than the traditional Sn‐Pb system. Sn‐Ag‐Cu seems to be a suitable soldering material.     

Electrochemical studies on the effect of (2E)‐3‐amino‐2‐phenylazo‐but‐2‐enenitrile and its derivative on the behaviour of copper in nitric acid

The objective of this work is to provide additional insight on the influence of (2E)‐3‐amino‐2‐phenylazo‐but‐2‐enenitrile and its derivative as corrosion inhibitors for copper in 0.5 M HNO₃. Electrochemical techniques (potentiodynamic polarization, polarization resistance and impedance spectroscopy) as well as weight loss measurements have been employed to study the corrosion inhibition. The investigated compounds have shown inhibition efficiency in 0.5 M HNO₃. Inhibition efficiency of these compounds has been found to vary with the concentrations of the compounds. The adsorption of these compounds on the copper surface from the acid solution has been found to obey Langmuir adsorption isotherm. The results revealed that the compounds are mixed type inhibitors. The effect of temperature on the inhibition efficiency was studied.     

New surfactants N‐alkyl‐N,N‐dimethyl‐N‐(3‐thienylmethylene)ammonium bromides: Preparation and anticorrosion evaluation

A new series of N‐alkyl‐N,N‐dimethyl‐N‐(3‐thienylmethylene)ammonium bromides (designated as NTA‐n, n = 8, 10, 12, 14, and 16) were synthesized. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization showed that NTA‐n could protect availably X70 steel from attack of HCl with an inhibition efficiency of about 95%. And the inhibition ability follows the following sequence: NTA‐8 < NTA‐10 < NTA‐12 < NTA‐14 < NTA‐16, suggesting that prolonging alkyl chain may enhance the inhibition capacity of the molecules. Theoretical investigation backs up well the experiment results. The correlation of theoretical calculation with the experiment results illustrated clearly double logarithmic relations between the corrosion current and the computed quantum chemical parameters such as the highest occupied molecular orbital energy (), energy gap (ΔE), molecular volume ( ν), electronegativity ( χ), the total energy of iron crystal with the adsorbed inhibitor molecule ), the interaction energy of Fe (1 0 0) surface and inhibitor molecules (), while an equation resembling Hammet relation was existed between the corrosion currents and the fraction of electrons transfer (ΔN). These linear correlations suggest the relation of the inhibitor structure with its inhibition activity: long alkyl chain and electron donating groups in NTA‐n will enhance the corrosion efficiency.     

An in‐situ FT‐IR investigation of the anodic behaviour of WC‐Co hardmetal

An in‐situ FT‐IR spectroelectrochemical and voltammetric study of the corrosion of a WC‐Co hardmetal grade is reported in this communication. A slightly acidic sulphate solution is considered. The potential‐ and time‐dependent anodic formation of CO₂ and Co(OH)₂ can be followed with the variation of the vibrational spectra. The formation of CO₂ occurs at the same potential at which tungsten oxidation starts to be detected and the breakdown of passivity can be observed by electrochemical methods. Our results suggest that a threshold potential exists for the oxidation of WC in WC‐Co, giving rise to the concomitant oxidation of tungsten and carbon at more noble potentials than those typical for the oxidation of the individual elements.     

Determination of the weaker phase in the pitting corrosion of non‐standard low‐Ni high‐Mn‐N duplex stainless steels

In this paper, the use of Energy Dispersive Spectrometry (EDS) is proposed to determine the partition coefficients of the elements of a new family of duplex stainless steels that are characterized by having low contents of nickel, together with high levels of manganese and nitrogen. From the values of the partition coefficients, the chemical compositions of the constituting phases have been determined, in order subsequently to calculate the value of the Pitting Resistance Equivalent Number (PREN) of each phase. The proposition put forward in this study is that the phase having the lower PREN determines the pitting corrosion behaviour of these types of steels. Results obtained by means of optical and scanning electron microscopy have provided confirmation that the pitting corrosion behaviour of these new materials gets determined by the resistance of the weaker phase and consequently by the phase having the lower PREN value. Lastly it has been proved possible to determine the existence of an exponential relationship between the alloys pitting potential (Ep) and the weaker phase PREN; this can be utilized for the low‐nickel duplex stainless steels design in which the pitting corrosion resistance is controlled.     

Einfluss der Halbzeugform auf die Korrosionsbeständigkeit hochlegierter Ni‐Cr‐Mo‐Werkstoffe

Effect of semi‐finished products on the corrosion resistance of high‐alloyed Ni‐Cr‐Mo materials The corrosion resistance of different semi‐finished products of six superaustenitic steels and nickel based alloys in the condition of delivery was investigated in some typical standard corrosion tests. The resistance of sheets, plates, strips, seamless tubes and welded tubes to intercrystalline corrosion was tested according to ASTM G 28 methods A and B, as well the resistance to pitting corrosion according to ASTM G 48 method C. The nickel based alloys 625, C‐276 and alloy 59 are resistant to the FeCl₃‐test according to ASTM G 48 method C and therefore a differentiation of these types in regard to their localized corrosion resistance was achieved only in the more aggressive ‘Green‐Death’‐solution. The laboratory experiments confirmed that the corrosion resistance is identical for all semi‐finished products and that it shows only a slight dependence of the surface condition of the materials tested. Additionally, some typical industrial and practical applications of the six high performance materials are presented to demonstrate the excellent corrosion resistance in the manufactured condition.     

Sulfate‐reducing bacteria‐assisted hydrogen‐induced stress cracking of 2205 duplex stainless steels

The paper presents the results of a laboratory investigation of the microbiologically assisted hydrogen‐induced stress cracking (HISC) of 2,205 duplex stainless steel (DSS). The testing of susceptibility toward HISC was performed with two different methods. Precharged in sulfate‐reducing bacteria (SRB), inoculated medium samples were subjected to slow strain‐rate testing in artificial seawater. In situ constant load tests were performed directly in SRB‐inoculated medium under hydrogen charging at 70% of the ultimate tensile strength. Samples tested in the biotic (SRB) conditions showed a considerable loss of ductility as compared to those tested in sterile conditions. Quantitative characteristics of fracture surfaces indicated increased susceptibility to HISC of biotic samples, therefore, suggesting a role of SRB in promoting hydrogen damage of DSS.     

X‐ray micro‐computed tomography analysis of accumulated corrosion products in deep‐water shipwrecks

Accumulated corrosion products from two different shipwrecks which had lain on the seabed (2.5 km depth) for 73 years were systematically analysed by three‐dimensional imaging at high resolution using X‐ray micro‐computed tomography. Complementary surface and chemical characterization experiments were conducted to identify the morphological structure of the corrosion products. Goethite was observed as the main corrosion phase found in both the wreck's corrosion products. However, other corrosion products such as silica, lepidocrocite, maghemite, magnetite, benyacarite, jarosite and amorphous materials were noticed using Fourier transform infrared spectroscopy, Raman spectroscopy and X‐ray diffraction. In addition, mineralized microbial structures were also observed as significant constituents of the corrosion products. However, there were significant differences between samples from the two shipwrecks including porosity, distribution and volume percent of the corrosion products components. The mechanism of different corrosion products formation was proposed and discussed in detail.