Influence of chromium content on corrosion resistance of low alloyed chromium steels in hydrofluoric acid and in other acid environments

Abstract

The influence of chromium content on the corrosion resistance of low alloyed chromium steels is studied in 1·7M (3%) HF, 1·4M (5%) HCl, 0·75M H₂S0₄, and 1·5M HClO₄ at 45°C. The presence of chromium in the steels decreases their corrosion resistance in HF solutions, and to a lesser extent in H₂SO₄ and HClO₄ acid solutions. There was little effect in the HCl solution.     

Potentiodynamic polarization behaviour of two 17Cr13Ni2·5 Mo austenitic steels with different N contents

Potentiodynamic polarization measurements have been made on two 17Cr13Ni2·5Mo austenitic steels differing from each other in the content of N. The electrolyte solutions have been 1·0M H₂SO₄, mixtures of HCl and H₂SO₄ and 1·0M HCl at 20°C. In 0·9 and 1·0M HCl the polarization curves of the two austenitic steels possess two peaks which are difficult to explain. Experiments indicate that the steel containing N is less corrosion resistant than the steel without N.     

Effects of Cu,Mo and C on the corrosion of deformed 18Cr8Ni stainless steels in H2SO4/NaCl solutions

The effects of Mo, Cu and C on the rate and type of corrosion of 18Cr8Ni stainless steels after deformation into U-shaped specimens have been investigated in 5N H₂SO₄ solutions containing up to 0·4M NaCl at 40°C. General corrosion, intergranular corrosion, and/or transgranular stress-corrosion cracking were found to occur, the type of attack depending on the contents of Mo, Cu and C in the steel, the concentration of NaCl in the solution, and the degree of deformation. In particular, it was observed that carbon has an effect on intergranular corrosion in these solutions, which is quite different from that observed in Strauss and Huey tests.     

Corrosion properties of nickel free austenitic stainless steels in 0·5M H2SO4 and 0·5M H2SO4 plus 0·4M NaCl

Abstract

The corrosion behaviour of four nickel free austenitic stainless steels were investigated in 0·5M H₂SO₄ and 0·5M H₂SO₄ plus 0·4M NaCl solutions by means of potentiodynamic and potentiostatic anodic polarisation testing. The performances of the nickel free alloys are compared to those of an experimental intermediate nickel alloy (4%Ni) and a standard AISI 304 steel grade. Once passivity was reached all alloys displayed similar current densities i _p in 0·5M H₂SO₄, independent from alloying. Mo and Cu were shown to be beneficial in decreasing the active dissolution currents and i _crit values. The commercial AISI 304 steel displayed superior resistance to pit initiation during potentiodynamic testing, and AISI 304 steel displayed the highest E _pit value of all alloys tested. When tested potentiostatically the N and Mo alloyed nickel free alloys showed excellent resistance to pit initiation and growth. The dominant effect of N was associated with repassivation of incipient pits, while Mo appeared to act at an earlier stage, suppressing initiation.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

Effect of chloride ions on corrosion behaviour of austenitic nickel and nickel free stainless steels in phosphoric acid solutions

The effect of chloride ions' presence (0·005–1·0M NaCl) in phosphoric acid solutions (5, 40 and 75%) on the corrosion behaviour of three austenitic stainless steels (an experimental steel Fe–18Cr–12Mn–0·6N and two trade grades, Fe–18Cr–9Ni and Fe–14Cr–15Mn–0·2N) has been studied by potentiodynamic polarisation measurements. The surface examinations of the samples tested involved X-ray photoelectron spectroscopy as well as optical and scanning electron microscopy. It was established that chlorides added to phosphoric acid solutions deteriorate the general corrosion resistance, and under anodic polarisation, they provoke pitting corrosion. The composition of the stainless steels significantly influences its corrosion behaviour in the phosphoric acid solutions containing chloride ions. The replacement of nickel with manganese and nitrogen on top of lower chromium content has a strong negative effect on the corrosion resistance.     

Investigation of phase composition of high temperature chromium steels and chromonitriding process in austenitic alloys

Abstract

The creep resistance of 9% chromium high temperature steels is determined by the alloy content and structure resulting from heat treatment. This paper describes the analysis of the phase composition using CALPHAD numerical modelling methods, of known modifications of 9% high temperature chromium steel: 9Cr–0·1C, 0·9Mo, 0·21V, 0·1Nb, 0·04N (P91) and 9Cr–0·1C, 2W, 0·5Mo, 0·21V, 005Nb, 0·05N, 0·005B (P92). The effect of alloying elements on the phase composition of the steel and the mutual effect of the composition on the nature and quantity of the phases Me23C6 and Me(CN) in the temperature range 570–620°C is described. On the basis of calculated data and experimental results, a composition for new high temperature steels with additional Co alloying (up to 3%) and varying carbon contents in the range 0·02–0·10% is proposed. Results are shown for investigations on high temperature chromium steels containing cobalt, including: effect of complex alloying with tungsten, molybdenum, and cobalt on the service properties and structural composition of steels; heat treatment processes for alloy variants and kinetics of structural change during creep and prolonged thermal aging. Data have been obtained comparing calculated and experimental data for the phase composition in chromium steel, also the effect of the phase composition on creep characteristics. On the basis of a complex laboratory investigation and industrial pilot heats, optimal composition variants for the alloy content of high temperature Cr–Mo–V–Co steels have been determined for practical applications. CNIITMASH has developed a chromonitriding technology for improving the corrosion, wear and scratch behaviour, as well as for protection against self-welding and other service characteristics of austenitic steel components and nickel alloys. Chromonitriding technology includes saturation of the component surface with chromium and nitrogen. The technology is intended for strengthening valves and bushings, water pump components, and components operating in liquid metal, burnt fuel residue, and other aggressive environments. The conditions governing the formation of the strengthening layers, consisting of an austenitic matrix (γ solid solution) and containing Cr₂N with a depth of up to 250 μm and a hardness of 750–950 HV have been determined. Thermodynamic analysis of phase formation conditions during the chromising and subsequent nitriding process over a wide range of temperatures and saturating media has been carried out. The technology has been optimised for process and media composition leading to a structure with maximum surface properties.     

Rapid Obtaining of Nano-Hydroxyapatite Bioactive Films on NiTi Shape Memory Alloy by Electrodeposition Process

Nano-hydroxyapatite (n-HA) crystalline films have been developed in this study by electrodeposition method on NiTi shape memory alloy (SMA). The electrodeposition of the n-HA films was carried out using 0.042 mol/L Ca(NO₃)₂ · 4H₂O + 0.025 mol/L (NH₄) · 2HPO₄ electrolytes by applying a constant potential of ?2.0 V for 120 min and keeping the solution temperature at 70 °C. The characterization of n-HA films is of special importance since bioactive properties related to n-HA have been directly identified with its specific composition and crystalline structure. AFM, XRD, EDX, FEG-SEM and Raman spectroscopy shows a homogeneous film, with high crystallinity, special composition, and bioactivity properties (Ca/P = 1.93) of n-HA on NiTi SMA surfaces. The n-HA coating with special structure would benefit the use of NiTi alloy in orthopedic applications.     

Studies on magnesium and its alloys in battery electrolytes

Abstract

Studies on the anode efficiency of magnesium and its alloys in battery electrolytes such as magnesium perchlorate, magnesium bromide, and magnesium chloride solutions were made by galvanostatic polarisation and corrosion rate measurements. The open circuit potential and corrosion rate of magnesium increase when lead is present in the alloy. The corrosion rates were observed to follow the order Mg> AP65 (Mg?(6·0?7·0) Al?(4·4?S·0)Pb) > AZ61(Mg?(5·8?7·2)Al?(0·5?1·5)Zn) > AZ31(Mg?(2·5?3·5)Al?(0·6?1·4)Zn) in all three electrolytes. Anode efficiency increased with increasing current density up to 40 mA cm^?2, but decreased above this level. Galvanostatic polarisation results indicated that corrosion of magnesium and its alloys in these electrolytes occurs under cathodic control. Of the tested alloys, AZ31 and AZ61 were found to be most suitable in respect of corrosion rate and anode efficiency in 2M Mg(ClO₄)₂ solution.     

Microstructural characterisation and corrosion behaviour of electroless Ni–W–P deposits of mixed nanocrystalline/amorphous structures

Abstract

An investigation has been undertaken of the influence of the mixed nanocrystalline and amorphous microstructure on the corrosion behaviour of electroless Ni–5·5W–6·5P (wt-%) deposits on steel substrates. The effects of annealing temperature on microstructure evolution were investigated. The corrosion behaviour of the deposits was evaluated by potentiodynamic polarisation in 0·5M H₂SO₄ solution and EIS measurements in 3·5%NaCl solution. Relationships between the microstructure and corrosion mechanisms of the as plated and the annealed deposits were considered by reference to microstructural information, including degree of crystallisation, grain sizes of both nickel and Ni₃P phases, porosity development, microstrains and residual stresses.     

Characteristics of nickel sulphide reduction in Ni2+ free background and Watts nickel plating electrolytes

Abstract

The electrochemical behaviour and composition of nickel sulphide coatings deposited on glassy carbon (GC) electrode by the successive ionic layer adsorption and reaction (SILAR) method using a nickel(II) ammonia complex and Na₂S solutions have been studied in Ni²⁺ free background, Watts nickel plating and 0·05 M H₂SO₄ electrolytes by the cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) methods. Analysis of XPS data suggests that the coating is a mixture of two sulphides where NiS dominates. In the Ni²⁺ free background solution at first the electrochemical reduction of the sulphur rich nickel sulphide to NiS occurs. After that the NiS to metallic Ni is reduced in the potential range of H₂ evolution. During the cathodic reduction of the coating in Watts nickel plating electrolyte the sulphur is reduced in the potential range from 0·0 to ?0·4 V, while at the potential values <?0·5 V the sulphur rich nickel sulphide and NiS reduction in the H₂ evolution range and Ni plating occur.     

Stress-induced failure of high-strength steels in environments containing hydrogen sulphide

Abstract

The pre-cracked cantilever beam test is a sensitive means of evaluating the relative susceptibilities of different steels to stress-Induced failure in H₂S-containing environments. This, test indicated that a 12Ni 5Cr.3Mo maragmg stee1 is considerably more resistant to stress-induced crackining than the low-alloy steels 0Ni 1Cr 2Mo and 2Ni 1Cr 3Mo. For low-alloy steels with no applied potential the failure appears to be caused predominantly by hydrogen embrittlement. Impressed anodic or cathodic potentials have no effect on the time to Failure of low-alloy steels where as a zone of immunity exists for maraging steel with Impressed potentials Within the approximate range ?0·6 to ?1·0 V. This suggests that the maraging steel is less susceptible to hydrogen embrittlement than the quenched and tempered low-alloy steels. Permeability studies indicate that stress-induced failure is related to the ability of the steel to transmit hydrogen and to the nature of hydrogen traps in the steel.     

The effects of low frequency load cycles on crack initiation in low alloy steels

The stress corrosion behaviour of low alloy steels was investigated with low frequency cyclic loading in 2 M (NH₄)₂CO₃ at 70°C. The tests were carried out using triangular and sawtooth load cycles with a constant critical strain-rate prevailing within periods of increasing stress irrespective of frequency. Since frequency changes with stress amplitude when using equal ?-values, the effects of frequency were also determined. The experiments were concentrated on crack initiation and initial crack propagation. The results show that the threshold stress which causes stress corrosion cracking (SCC) in ammonium carbonate solution is considerably reduced for all steels tested during low-frequency load cycles. In contrast to tests with constant load, propagating stress corrosion cracks can be initiated below the lower yield strength. The effects of stress/strain amplitude and of the magnitude of deformation on crack propagation were determined.     

Influence of inclusion and grain boundary on corrosion behaviour of low alloy steel in cargo oil tank bottom plate environment

Corrosion behaviours of low alloy steels with various alloying element contents were studied in simulated corrosion environment (pH?=?0·85, 10% NaCl solution) of cargo oil tank (COT) bottom plate corrosion environment. Corrosion rate was measured, and the surface morphology of test steels was characterised using scanning electron microscope (SEM). The electrochemical results were investigated using polarisation curves and electrochemical impedance spectroscopy (EIS). The corrosion behaviour of inclusion was investigated using in situ SEM images and energy disperse spectroscopy (EDS) analysis. The effect of misorientation distribution on corrosion resistance of steel was evaluated using electron backscattered diffraction microscopy. The results showed that MnS dissolved into the simulated COT corrosion solution before steel matrix, whereas TiO₂ would not be dissolved. A 10–20 μm area was formed as a circular cathode around inclusion centre. The corrosion was inclined to occur in the place where curvature radius of inclusion is small. An increase in the proportion of low energy grain boundaries was conducive to the improvement of corrosion resistance especially in the early stages of corrosion.     

Studies on copper-semiconducting layer-electrolyte systems—I. Electrode potentials of copper and electrodeposited copper sulphide in S2− and Cu2+ media

Copper sulphide layers electrodeposited from a Na₂S bath consist chiefly of cuprous sulphide. Calculated values of the specific conductance of the deposited sulphide average around 3·8 × 10^?5 ohm^?1.cm^?1.Electrode potentials, E_H, of the Cu/Cu_2?δS in unbuffered Na₂S solutions and in solutions buffered at pH 9, correspond to the behaviour of a Cu/Cu_2?δS, overlayered with CuS/electrolyte. Steady reproducible potentials are obtained with the above electrodes in 0·5–10^?3M CuSO₄ solutions, closely comparable with those of the reversible Cu/Cu²⁺ couple.     

Spot weldability of TRIP assisted steels with high carbon and aluminium contents

Abstract

We examine here the spot welding characteristics of transformation induced plasticity assisted steels, which contain δ-ferrite as a consequence of their aluminium concentrations of 3·5 or 5·6 wt-% and which also have high carbon contents of 0·3 or 0·4 wt-% when compared with conventional automotive steels. The resistance spot welds are tested in both shear and cross-tensile tests in order to determine the so called ductility ratio, which is a parameter associated with the fitness of such welds for automotive applications. With an increase in the δ-ferrite fraction from 0·19 to 0·5, the hardness variation across the weld and heat affected zone is decreased approximately from 400 to 150 HV. It seems that the presence of stable δ-ferrite is helpful in reducing hardness variations and in achieving a significant ductility ratio of 0·39.     

The electrochemical polishing behaviour of P/M high-speed steel (ASP 23) in perchloric-acetic mixed acids

The electropolishing behaviour of P/M high-speed tool steel (ASP 23) has been investigated in HClO₄-CH₃COOH mixed acids at a temperature range from −10 to 30 °C. The surface morphologies and roughness of polished samples were examined with field-emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The results show that the steel can be electrochemically polished in the mixed acids with concentrations of HClO₄ from 20 to 80 vol%. Temper-induced carbides in both MC and M₆C forms can be leveled together with the steel matrix when the sample was polished at lower temperatures (<0 °C) in the mixed acids with higher HClO₄ contents (60-80 vol%), and the roughness (Ra) of the polished surface can be achieved to 30-50 nm. The electropolishing behaviour of the steel is under mass-transfer control and the anion ClO₄⁻, and/or its complexes, suggested to the governing species for the acceptor-limited mechanism during anodic dissolution in limiting-current plateau.     

Slurry Erosive Wear Evaluation of HVOF-Spray Cr2O3 Coating on Some Turbine Steels

In this study, Cr₂O₃ coatings were deposited on CF8M and CA6NM turbine steels by high-velocity oxy-fuel (HVOF)-spray process and analyzed with regard to their performance under slurry erosion conditions. High Speed Erosion Test Rig was used for slurry erosion tests, and the effects of three parameters, namely, average particle size, speed (rpm), and slurry concentration on slurry erosion of these materials were investigated. SEM micrographs on the surface of samples, before and after slurry erosion tests, were taken to study the erosion mechanism. For the uncoated steels, CA6NM steel showed better erosion resistance in comparison with CF8M steel. The HVOF-sprayed Cr₂O₃-coated CF8M and CA6NM steels showed better slurry erosion resistance in comparison with their uncoated counterparts. It may be due to the higher hardness as a result of HVOF-sprayed Cr₂O₃ coating in comparison with the uncoated CF8M and CA6NM steels.     

Formation and Breakdown of Surface Films on Copper in Sodium Hydrogen Carbonate and Sodium Chloride Solutions: II. Effects of temperature and pH

Abstract

The anodic behaviour of copper in air-saturated solutions at 25°, 60° and 90°c has been determined by stepwise potentiortatic polarisation. In 0·01 M NaHCO₃, the critical breakdown potential of the protective oxide layer is displaced to considerably more positive potentials as the temperature increases. In 0·01 M NaHCO₃ containing 0·01 M NaCl, the critical breakdown potential is displiiced tornore negative values with increasing temperature, but the corrosion potential–breakdown potential difference undergoes little change. In 0·01 M NaCl, the surface film on copper is essentially non-protective, and linear Tafelplots are observed. A change in the pH of 0·01 M NaHCO₃, with or without 0·01 M NaCl, from 8·6 to 7·3 has little effect on the valuesof the breakdown potentials. The significance of the results in relation to the initiation of pitting corrosion of copper in natural waters is discussed.     

Effectiveness of pretreatment method to hinder rebar corrosion in concrete

Abstract

This paper aims to evaluate the ability of phosphate pretreatments applied on steel rebars to hinder the corrosion reinforcements using synthetic pore electrolyte and mortar contaminated by chloride ions. The electrochemical behaviour of the pretreated substrate was assessed by corrosion potential, polarisation resistance and electrochemical impedance spectroscopy measurements. The results have demonstrated that the treatment of the rebar by immersion in the Na₃PO₄ (0·5M) solution favours the formation of a passive layer on the steel rebar surface, which increases the resistance to corrosion initiation up to 0·3M Cl^– instead of 0·1M Cl^– without treatment. The pretreatment also provides enhancement of corrosion protection of the steel rebar in mortar. The evolution of the impedance spectra in function of chloride concentration is in a fairly good agreement with the results obtained from R_P measurements.