The aspect ratio of surface grooves and metastable pitting of stainless steel

The metastable pitting behavior of 316L stainless steel with different surface roughness in 0.01 M NaCl solution was studied using potentiostatic and potentiodynamic polarization methods and statistical treatments. As surface roughness decreased, the nucleation rate of metastable pits decreased. The peak current of metastable pits also decreased, but the average growth rate of metastable pits showed a slight increase. The aspect ratio of surface groove, w/d, was suggested to characterize the openness of surface grooves, in which w is the width of the groove at the opening and d the depth of the groove. Bigger w/d value indicates more smooth surface, and it is more difficult for micro-pit to nucleate. Under certain conditions of solution and potential, there is a critical w/d value. Metastable pits may nucleate only on those surfaces whose w/d values are less than the critical value. As the aspect ratio w/d increases, the nucleation rate of metastable pits decreases linearly, and both metastable pitting potential E_m and pitting potential E_p move in the positive direction.     

Effect of surface polishing on SCC susceptibility of sensitised type 304 stainless steel

Abstract

The effect of surface polishing on the stress corrosion cracking (SCC) susceptibility of sensitised SUS 304 stainless steel was studied by using the well controlled surface polishing method which has been developed for the surface preparation of the semiconductor material. The SCC susceptibility of the polished specimen was examined by using the slow strain rate testing method, at which the specimen was kept at a constant potential in 0·1M Na₂SO₄ solution kept at 368 K. The most effective suppression effect on the SCC susceptibility was found in the case of surface polishing using alumina slurry, followed by water soluble fullerene and diamond slurry, compared with the mechanical polishing with #0/6 paper. Surface roughness and pitting potential of polished surface were compared with the SCC susceptibility of polished sensitised SUS 304 stainless steel. It was found that the SCC susceptibility was decided mainly by the roughness factor rather than pitting resistance. Experimental evidences suggest that the SCC susceptibility was decided by the crack initiation being controlled by surface roughness.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Slurry erosion-corrosion characteristics of squeeze cast aluminum alloy-aluminum borate composite

The slurry erosion-corrosion behavior of 19.5 vol.% Al₁₈B₄O₃₃ whisker reinforced AC4C Al composite in aqueous slurry containing 3.5 wt.% NaCl and 20 wt.% silica has been investigated using a jet-in-slit rig. Erosion-corrosion tests were performed with slurry having a jet velocity range 6.4 ms⁻¹ to 15.2 ms⁻¹ and at normal impact angle. The results indicated that the slurry erosion-corrosion characteristics were affected by the slurry velocity and material properties. At low slurry velocities, as compared to the unreinforced alloy, the erosion-corrosion resistance of the composite was improved because of increase in hardness as a result of whisker addition. However, steady-state erosion-corrosion rate was found to increase with the addition of whisker to the matrix alloy at high slurry velocities. The erosion-corrosion rates of materials in the saline slurry were much higher than those in the water slurry even though the corrosion components were very small. It is found that the synergistic factor for the materials reduced with increasing slurry velocity although the synergism between erosion and corrosion was greater at high slurry velocity. Localized corrosion occuring in the strain-hardened layer caused a decrease in the fracture strain of composite during erosion-corrosion. In the saline slurry, the material removal was enhanced through cracking of flakes and detaching of whisker induced by stress and corrosion.     

Selective corrosion of 550 °C aged Cu10Ni–3Al–1.3Fe alloy in NaCl aqueous solution

This work studied the electrochemical behavior of a solution treated or 550 °C aged Cu10Ni–3Al–1.3Fe alloy, in 0.01 M NaCl aqueous solution, through potentiodynamic polarization in both stagnant condition or under erosion process. Results showed the occurrence of a passivity break potential (E_pb), related to the beginning of the denickelification process, which occurred as a localized attack under stagnant electrolyte. Under erosion conditions localized denickelification was not observed, despite of the presence of E_pb. This could indicate that selective corrosion of Ni, which caused the observed E_pb, occurred as a dissolution–redeposition process, with removal of the Cu deposits during erosion process.     

Determination of characteristic pitting potentials for aluminium by use of the potentiostatic methods

Potentiostatic methods were used to determine the pitting potential, E_p, and the protection potential against pitting, E_pp, for unalloyed aluminium in a de-aerated solution of 3 %NaCl at 0 and 30°C. The results show that the pitting potentials determined by these methods are more active than the pitting potentials determined by potentiokinetic and galvanokinetic methods.^1,2 The results also prove the existence of only one characteristic potential for pitting corrosion on aluminium as indicated in earlier papers. The temperature has a marked influence on the pitting potential which decreases with increasing temperature.     

Use of Potentiokinetic and Potentiostatic Methods for the Determination of Characteristic Potentials for Pitting Corrosion of an Fe–Cr Alloy

Abstract

Potentiokinetic and potentiostatic methods were used to determine the pitting potential, E_p, and the potential for protection against pitting, E_pp, for a 13 Cr steel in a deaerated solution of 3% NaCl. It was found that potentiokinetic methods gave pitting potentials that were far too noble and protection potentials that were far too base. Kinetic investigations by the potentiostatic method seem to offer the best likelihood of obtaining correct values for the pitting potential.     

Erosion testing and surface preparation using abrasive water-jetting

Erosion testing and surface preparation are studied using a 3-axis Computer Numerical Control (CNC) abrasive water jetting (AWJ) apparatus. The effects of erosion time t, impingement angle α and pressure p on the erosion rate E, average surface roughness R _a, and surface hardness Rockwell C Hardness (HRC) were investigated in detail. Compared with conventional grit blasting, AWJ can reduce grit embedment in the target material due to the action of the high-pressure water. AWJ also has the advantage of generating a higher average surface roughness R _a over water jetting (WJ) due to the action of abrasive particles. In addition, AWJ increases the surface hardness HRC of the substrate material. The obtained higher degree of average surface roughness is helpful for improving the bonding strength between the coating and the substrate material. The erosion testing and the surface preparation are numerically controlled by a 3-axis CNC system; therefore precise and detailed results for various operating parameters can be obtained.     

Study of the pitting corrosion of superduplex stainless steel and X-65 carbon steel during erosion–corrosion by cyclic polarisation technique

Erosion-enhanced corrosion behaviour of X-65 carbon steel and UNS S32750 superduplex stainless steel was investigated by electrochemical cyclic polarisation. The tests were performed using a jet slurry device coupled with a potentio-galvanostat at various jet velocities of 4, 6.5 and 9?m?s^?1 and impingement angles of 30 and 90? in a 3.5?wt-% NaCl water containing 6?wt-% silica sand particles. The results showed that increasing the jet velocity and impingement angle increased the corrosion rate of both alloys. Negative hysteresis and greater E_rp than OCP were observed for superduplex stainless steel in all erosion–corrosion conditions that indicated the pitting resistance of the alloy. However, the low resistance of carbon steel against pitting during erosion–corrosion was demonstrated by positive hysteresis in the cyclic polarisation curves as well as SEM images of the eroded surfaces.     

Effect of Plastic Deformation on the Corrosion Behavior of a Super-Duplex Stainless Steel

The role of plastic deformation on the corrosion behavior of a 25Cr-7Ni super-duplex stainless steel (SDSS) in a 3.5 wt.% sodium chloride solution at 90 °C was investigated. Different levels of plastic strain between 4 and 16% were applied to solution annealed tensile specimens and the effect on the pitting potential measured using potentiodynamic electrochemical techniques. A nonlinear relationship between the pitting potential and the plastic strain was recorded, with 8 and 16% causing a significant reduction in average E _p, but 4 and 12% causing no significant change when compared with the solution-annealed specimens. The corrosion morphology revealed galvanic interaction between the anodic ferrite and the cathodic austenite causing preferential dissolution of the ferrite. Mixed potential theory and the changing surface areas of the two phases caused by the plastic deformation structures explain the reductions in pitting potential at certain critical plastic strain levels. End-users and manufacturers should evaluate the corrosion behavior of specific cold-worked duplex and SDSSs using their as-produced surface finishes assessing in-service corrosion performance.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrate solutions

The anodic behaviour and SCC susceptibility of pure copper and four α-brasses of different zinc alloy concentration in a 1 M NaNO₃ solution was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. SCC was exclusively observed when the potential was equal to or higher than a certain critical value (E_c) at which pitting initiated under slow dynamic straining. It is concluded that the same SCC mechanism should be operating during SCC of copper and α-brasses in sodium nitrate, sodium nitrite and copper (II) nitrate solutions.     

Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

Repassivation of the damage generated by cavitation on UNS N08031 in a LiBr solution by means of electrochemical techniques and Confocal Laser Scanning Microscopy

The objective of this work is to study the influence of cavitation on the corrosion behaviour of Alloy 31, a highly-alloyed austenitic stainless steel (UNS N08031), in a LiBr heavy brine solution (992 g/L) at 25 °C. The presence of cavitation shifted the OCP value towards the active direction by 708 mV_Ag/AgCl, increased anodic current densities and passivation current density, i_p, and reduced the pitting potential, E_p.Repassivation behaviour of Alloy 31 has been investigated by using potentiostatic tests at different potentials. The current density transient obtained after interrupting cavitation was used to obtain the repassivation index, n, provided by the slope of the log i(t) vs. log t representation. The value of n decreased as the applied potential was increased, reaching values near zero for potentials close to the pitting potential. The damage generated during the potentiostatic tests has been quantified by means of Confocal Laser Scanning Microscopy.     

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, E_pit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, E_ocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to E_pit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.     

Effect of velocity and elevated temperature on pitting of Al 6013-20SiCp composite in 3·5 wt-%NaCl

Abstract

This paper presents new data on the effect of temperature and velocity on the pitting potential E_p, corrosion potential E_corr, and pitting morphology of O, F, and T4 tempers of alloy 6013-20SiC_p in 3·5 wt-%NaCl in the temperature range 30–130°C and at velocities of 0·5–1·0 m S_?1. The pitting potential of tempers T4 and O tended to increase with increasing temperature and flow velocity, but the pitting potential of temper F showed the opposite behaviour. Temper T4 of the alloy showed the best resistance to pitting. Alloy 6013-20SiC_p in temper T4 is therefore considered to be suitable for application in chloride containing environments.     

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

The variation with time of the open circuit potential of high molybdenum containing stainless steel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range 30-100 °C. The plot of steady state potentials as function of temperature showed an inflection at 50 °C, attributed to the decrease of oxygen solubility in test solution above 50 °C. Potentiodynamic cycling anodic polarization technique was used to determine the critical pitting potential (E_pit) and the critical protection potential (E_prot) of the steel in 4-30% NaCl solutions at temperatures between 30 and 100 °C. By plotting the two values versus solution temperature, the corresponding critical pitting (CPT) and the critical protection (CPrT) temperatures were determined. Both parameters decreased with increasing chloride content. Above the CPT, E_pit and E_prot decreased linearly with log[Cl⁻]. The addition of bromide ions to the solution shifted both E_pit and E_prot towards positive values. In 4% NaCl, E_pit increased linearly with pH in the range 1-10. The combined effect of chloride ion concentration and pH on the morphology of the pits was examined by scanning electron microscopy (SEM) following potentiodynamic cycling anodic polarization.     

Surface analytical and electrochemical study on the role of adsorbed chloride ions in corrosion of stainless steels

The pitting potential E_pit of 18/8 CrNi stainless steels with different sulfur content (0.003, 0.017 and 0.29%) has been determined from potentiodynamic polarization curves in deaerated neutral solutions (0.1 M NaCl and 1 M Na₂SO₄ + 0.1 M NaCl) with nominally identical chloride content. E_pit decreased with increasing sulfur content of the alloy and was about 0.2 V more negative in pure 0.1 M NaCl solution. The chemical composition of the passive film and the adsorbed chloride content have been determined by XPS surface analysis on mechanically polished samples passivated for one hour at potentials below the pitting potential. XPS results show that the surface films are composed in all cases of mixed iron‐chromium oxi‐hydroxides with a higher chromium content than the bulk composition. The average passive film composition (ca. 40% chromium oxi‐hydroxide) and the film thickness (2.3 ± 0.2 nm) were similar for all 18/8 CrNi steels regardless the different sulfur content. The amount of chlorides in the passive film is about twice as high on steels exposed to pure 0.1 N NaCl solution compared to the mixed 1 M Na₂SO₄ + 0.1 M NaCl solution. Thus the lower pitting potential measured in pure 0.1 N NaCl solution correlates with the higher amount of chloride ions on the passive film surface.     

Pitting corrosion of artificially aged T6 AA2024/SiCp composites in 3.5 wt.% NaCl aqueous solution

The pitting corrosion behavior of the underaged (UA), peakaged (PA) and overaged (OA) T6 AA2024/0, 8, 14, 19, 24 vol.% 40 μm SiC_p(particles) composites was studied. The processing route used for the materials was the compocasting technique. Corrosion potentials (E_cor), pitting potentials (E_pit) as well as protection potentials (E_prot) were extracted from Double Cycle Polarization (DCP) curves contacted in aerated 3.5 wt.% NaCl aqueous solution. In addition 40 days immersion tests carried out and weight loss curves as well as total pit depth measurements were acquired. Pitting initiation and propagation as the main corrosion mechanism was driven by the aging kinetics which is ruled by the reduction in the retained vacancy concentration and at the same time by the increase in dislocation density as SiC_p volume fraction increases. Thus, alteration in pitting behavior among composites of different SiC_p content took place, although their ageing status was exactly the same.     

Characterization of Corrosion Product Layers from CO2 Corrosion of 13Cr Stainless Steel in Simulated Oilfield Solution

The influence of temperature and flow rate on the characterization and mechanisms of corrosion product layers from CO₂ corrosion of 13Cr stainless steel was carried out in simulated oilfield solution. Cyclic potentiodynamic polarization method as well as weight loss tests in autoclave were utilized to investigate pitting corrosion behavior at various temperatures. Weight loss tests were performed at 100 and 160 °C under dynamic and static flow conditions. At the same time, the significant pitting parameters such as E _corr, E _pit, E _pp, ∆E, and I _pass in cyclic polarization curves at various temperatures were analyzed and compared for revealing the pitting behavior of 13Cr stainless steel. The surface measurement techniques such as SEM, XRD, and XPS were used to detect the corrosion product layers. The results showed that both temperature and flow rate had significant effects on characterization of corrosion product layers or passive films formed on 13Cr stainless steel in CO₂ corrosion system. At high temperature, lots of pits were formed at the localized corrosion areas of metal surfaces. Corrosion rates under the condition of 5 m/s were higher than those under the static condition regardless of the test temperatures.     

Corrosion–erosion wear of N80 carbon steel and 316L stainless steel in saline–quartz slurry

The corrosion–erosion behavior of carbon steel N80 and austenitic stainless 316L are investigated both in water–quartz slurry and saline–quartz slurry. The sample mass losses after each test and the roughness of sample surfaces are measured and a potentiostat is used to gain the polarization curves. The worn surfaces and the corrosion–erosion wear mechanism are analyzed using scanning electron microscopy (SEM) and 3D ultra‐deep microscopy. Results show that the R_a for N80 is much larger than 316L under the same experimental condition, and the weight losses of the two materials show a similar trend except for tests with higher impact angles. The lowest R_sm for 316L matches along with the highest R_sm for N80 at the 45 min testing time. According to the obtained results of the corrosion–erosion mechanism of the N80 and 316L based on the SEM and 3D ultra‐deep microscopy, the conclusion is drawn that the ploughing and crashing are the major mechanisms for the forming of surface morphologies and the mass removal. Different impact angles and materials affect the corrosion–erosion process to some extent.