Crevice corrosion monitoring of titanium and its alloys using microelectrodes

Crevice corrosion of titanium and its alloys in 10% sodium chloride was investigated at 100°C with the aid of microelectrodes. Potential, pH and chloride ion concentration inside the crevice were monitored using an Ag/AgCl electrode, a tungsten microelectrode and a Ag/AgCl chloride ion selective microelectrode, respectively. The pH and Cl^? concentrations within the crevice were calculated from the standard potential‐pH and potential‐log[Cl^?] calibration curves. The effect of Mo on the crevice corrosion of titanium was also studied. The passivation behavior on the titanium and Ti‐15%Mo alloy was studied using electrochemical impedance studies. There was no apparent change in pH and Cl^? ion activity inside the crevice for the alloy at 100°C, whereas a marginal decrease in pH and increase in Cl^? ion concentration were observed for pure titanium. Thus pure titanium is susceptible to crevice corrosion in hot 10% NaCl solutions at 100°C. The chloride ion activity was found to be reduced for the alloy so that the pH inside the crevice increased. The corrosion reaction resistance (R_t) was found to increase with the addition of Mo as an alloying element. It also increases with externally applied anodic potential. Hence, Mo is an effective alloying element, which enhances the crevice corrosion resistance of titanium.     

Vorwort

Investigation into the pitting corrosion of passive austenitic CrNi steels in neutral chloride solutions Stainless steels of the 18/8 CrNi-Type suffer pitting corrosion by halogen ions. Potentiokinetic, galvanostatic and potentiostatic tests as well as the ferro-ferricyanide-tests showed that pitting susceptibility increases with Cl^? content, temperature and oxygen content of the electrolyte, with decreasing homogeneity and purity of the material. Cold-working is without significant influence on the pitting potential. Mn up to 11,2% increases pitting potential by 50 mV, Ni up to 25% increases the potential by 200 mV, Cr up to 30 and Mo up to 4,6% increase the potential by max. 900 mV in 3% NaCl of p_H 7,5 at 22° C. The four methods employed gave the same pitting potentials. Before arriving at the potential of stationary pitting all steels showed a region where formation and repassivation of single pits occur. Cathodic protection to suppress pitting causes H₂-absorption. The amount of absorbed H₂ increases as the potential becomes more negative. Hydrogen embrittlement was not observed. The absorbed H₂ impairs pitting resistance. The study of Cl^?-adsorption as a pitting releasing process by help of the potentiostatic method, working with a reference source of triangular alternating voltage gave no indication of a preferential Cl^?-adsorption or an Cl^?-adsorption-potential near the pitting potential.     

Untersuchungen zum Korrosionsverhalten nichtrostender Stähle in Phosphorsäure in Abhängigkeit von dem Chlorid- und Fluoridgehalt und einer Wärmebehandlung beim Schweißen

Investigations into the corrosion behaviour of stainless steels in phosphoric acid in dependence on the chloride and fluoride contents and a heat-treatment during welding In the present investigation the corrosion behaviour of recently developed high-alloyed austenitic (X 2 NiCrMoCU 25 20) and austenitic-ferritic (X 2 CrNiMoCuN 25 5) steel has been tested towards phosphoric acid, the composition and concentration of which was adjusted to correspond to crude phosphoric acid with varying contents of Cl^?, F^?, Fe³⁺, and SO₃. Current density-voltage curves were recorded on steel test-specimens at room temperature, 45°C, and 80°C. In addition, test-pieces of both steels, after annealing treatment between 800°C and 500°C, as well as welded specimens of similar type were examined for the effect of heat input and consequent structural alterations on the corrosion behaviour. Both steels exhibit good corrosion resistance towards pure phosphoric acid. Cl^? and F^? ion impurities render the acid more aggressive. Austenitie-ferritic steel reacts more sensitively than austenitic steel towards heat input, resulting in deterioration in corrosion resistance. This may be attributed to the precipitation of inter-metallic phases in the ferritic structure.     

Effect of Chloride and Fluoride on the Rate of Corrosion of dental amalgam alloys

The anodic behaviour of massive and dental amalgam alloys in artificial saliva containing different concentrations of Cl^? or F^? ions has been studied. Quantitative relations could be established between concentrations of these ions and the height of the peaks corresponding to anodic oxidation of Sn or Ag. Contrary to f^?, Cl^? ions from Ag complexes via AgCl which excess cl^?ions. The massive alloy is stable below 0.15 V. Calcium fluoride can be safely admixed with dental amalgam alloy without enhancing its corrosion.     

An investigation on the effect of bleaching environment on pitting corrosion and transpassive dissolution of 316 stainless steel

Pitting corrosion and transpassive dissolution of 316 stainless steel in a solution containing five percent of commercial bleaching liquid was investigated by employing potentiodynamic polarization method and recording corrosion potential during immersion. Today commercial bleaching liquids are widely used as a cleaner additive. Therefore those house appliances made from stainless steels are in contact with aqueous solution containing bleaching liquid. This may cause severe localized corrosion and transpassive dissolution. In order to investigate the possibility of tranpassive dissolution of stainless steel by bleaching liquid, potentiodynamic polarization and recording the variation of corrosion potential of specimens were carried out in 0.2 M Na₂SO₄ solution containing 5%wt. commercial bleaching liquid. A 500 mV drop in transpassive potential and also instantaneously ennobled corrosion potential revealed the possibility of transpassive dissolution due to the oxidizing effect of the species such as free chlorine and its derivatives in bleaching liquid. Evaluation of the occurrence of localized corrosion at the presence of Cl^? and bleaching liquid was investigated by similar electrochemical experiments in 0.2 M Na₂SO₄ + 0.4 M NaCl containing 5%wt. bleaching solution. Initiation of stable pitting at potentials lower than the transpassive potential as well as a sharp increase of the corrosion potential in this environment demonstrates the possibility of pitting corrosion.     

The behavior of chromium and molybdenum in the propagation process of localized corrosion of steels

In order to clarify the effect of Cr and Mo on the propagation of localized corrosion of steels, the electrochemical behavior of pure chromium and molybdenum and of some stainless steels was studied under conditions similar to those existing inside occluded corrosion cells of steels. Oxygen-free FeCl₂ and/or CrCl₃ solutions were used for experiments. The surface films formed in these conditions were analyzed by means of AES and XPS. Cr is passive if the pH value >ca. 1.8 and Cr may retard the propagation of localized corrosion of steels in its early stage. The hydrolysis of Cr³⁺ may change the pH to even lower values. At such low pH and high Cl^? concentrations, Cr is active in the potential range of interest for localized corrosion of steels. Mo is passive under these more acidic conditions and the passivity of Mo in this potential range is attributed to the formation of a film of MoO₂ (or hydrated oxide), which is extremely thin but very protective. Mo is thus effective to retard the propagation of localized corrosion of steels in its more advanced stage.     

Die Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Effects of Halides on the Corrosion of Mild Steel in Molten NaNO3-KNO3 Eutectic

Abstract

The effect of alkali halides on the corrosion behaviour of mild steel in, (Na,K)NO₃ eutectic has been studied at temperatures ranging from 300°–450°c. Steady-state potentials of the steel electrode vary with the concentration of Cl^?, Br^?, and I^? ions according to: E = a + b log C. Gravimetric; measurements show that the corrosion process of steel in the melt without and with halide ions, at concentrations ≤0·l M, is essentially the same, the weight gain being parabolic with time. At higher halide concentrations the corrosion rate is linear with time. The corrosion rate also increases with increase in temperature. Both potential and weight gain measurements show that aggressiveness increases in the order: Cl^?<Br^?<I^?. It is suggested that in this system two opposing forces compete on the steel surface. The oxide ions help in building or repairing a passivating film of Fe₃O₄, whilst the aggressive halide ions attack weak areas in the film and assist the corrosion process. The process which prevails depends on the relative concentrations of the two species.

In the case of fluoride, both the potential and weight gain of steel vary with time in an irregular manner, and no definite relation could be obtained. This has been attributed to the low solubility of fluoride in the melt and to the properties of the corrosion products, in that the film formed does not adhere to the metal surface.     

Influence of different heat treatments on corrosion resistance of martensitic stainless steels

The changes in microstructure, caused by different heat treatments, have considerable influence on the corrosion resistance of stainless steels. The heat treatment causes an alteration of carbide contained in steels. Changes in the constant B for these steels, (B = Rp · i_COR) have been observed in quenched, tempered and annealed conditions. Pitting corrosion resistance of martensitic stainless steels in quenched and tempered conditions in 0.1M H₂SO₄ by adding Cl^? ions has been investigated.     

The influence of alloying elements on the corrosion behaviour of ferritic steels in simulated combustion atmospheres

The low‐chromium steels (Cr content 0.7–1.4 wt.%) are usually being used in low temperature boiler applications where the environments are not so aggressive as in waste‐fire boilers. To improve their mechanical properties and corrosion resistance several alloying elements have been used. In the present study the new low‐chromium steels modified with Si, Ni and Ce have been investigated. These steels were compared with the common boiler steel – 2.25Cr1M. The multi‐sample exposure tests were made in moist air (8% O₂ + 15% H₂O) with and without 200 ppm SO₂ or 2000 ppm HCl addition at the isothermal temperature of 500°C. Corrosion products were analysed by Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM/EDS) and XRD techniques. The results of the study show that addition of Ni and Si to the steels studied could improve their corrosion resistance, but only in moist air atmospheres. In SO₂‐containing atmospheres the corrosion resistance seems to remain the same, when in HCl‐containing atmospheres Ni and Si additions seem to have a negative effect on the corrosion behaviour. As could by seen from SEM analysis, the scales formed, on the steels with above elements, are more adherent to the metallic core. The cerium was added to the studied materials mainly to improve their mechanical properties. This alloying element seems to increase the corrosion rate of materials, especially in atmospheres containing Cl.     

Zum Korrosionsverhalten von unlegierten und niedriglegierten Stählen in Kohlensäurelösungen

Corrosion of unalloyed and low alloyed steels in carbonic acid solutions In long period experiments unalloyed steels were found to corrode in O₂-free CO₂-saturated 0,5 M sodium sulfate solutions at 25° C with flow independent corrosion rates of 0,05–0,1 mm/a. Cold-work at unalloyed steels as well as higher contents of phosphorus, copper and chromium in unalloyed and low alloyed steels increased the corrosion rate to 0,2 to 1,4 mm/a. In O₂-free CO₂-saturated distilled water unalloyed steel showed a decreased resistance affording corrosion rates of 0,4 mm/a. Traces of oxygen as introduced by CO₂ containing 100 ppm O₂ increased the corrosion rate only above pH 4,2. All investigated unalloyed and low alloyed steels showed pitting corrosion after long induction periods. The shortest induction periods were observed in O₂-free CO₂-saturated distilled water. Sulfate ions and traces of oxygen seem to inhibit pitting corrosion.     

Influence of sulphide ions on corrosion resistance of special austenitic stainless steels in phosphoric acid

Abstract

The influence of sulphide ions on the corrosion resistance of some special stainless steels in 40 wt-% H₃PO₄ with 330 ppm SO^2?₄ and 1000 ppm Cl^? at 80°C has been investigated using electrochemical (polarisation curves) and spectroscopic (infrared, SIMS) techniques. The behaviour of ZI NCDU 25–20 and ZI CNDU 25–25 commonly used in the phosphoric acid industry is compared with that of ZI CN 25–20 to estimate the role of the alloying elements. The corrosive effect of sulphide ions is confirmed: they shift the corrosion potential to more negative values and increase the anodic current in the active and passive states. In order of increasing corrosion rate the three steels are ranked as follows: ZI CNDU 25–25, ZI NCDU 25–20, ZI CN 25–20. The corrosion products consist largely of magnetite in a more or less oxidised state with chromium and nickel substituted for iron. The film formed on the ZI CNDU 25–25 alloy is more protective because of the high chromium content of the mixed oxide and the effect of molybdenum. The film retards the diffusion of corrosive ions through to the metal andfacilitates the development of passivity. However, the presence of a sufficiently high concentration of sulphides in solution prevents the establishment of passivity.     

Electrochemical Corrosion Investigations on Anaerobic Treated Distillery Effluent

Present study is focused on the corrosivity of anaerobic treated distillery effluent and corrosion performance of mild steel and stainless steels. Accordingly, electrochemical polarization tests were performed in both treated distillery and synthetic effluents. Polarization tests were also performed in synthetic solutions and it was observed that Cl^? and K⁺ increase whereas SO₄ ^?, PO₄ ^?, NO₃ ^?, and NO₂ ^? decrease the corrosivity of effluent at alkaline pH. Further, comparison in corrosivity of distillery and synthetic effluents shows the former to be less corrosive and this is assigned due to the presence of amino acids and melanoidins. Mild steel experienced to have the highest corrosion rate followed by stainless steels—304L and 316L and lowest in case of SAF 2205. Relative corrosion resistance of stainless steels is observed to depend upon Cr, Mo, and N content.     

The use of microcapillary techniques to study the corrosion resistance of AZ91 magnesium alloy at the microscale

The AZ91 alloy is composed of Mg₁₇(Al, Zn)₁₂ precipitates, an eutectic phase around these precipitates, AlMn intermetallic particles and an α-Mg solid solution (matrix). The corrosion behaviour of AZ91 was investigated at the microscale by means of the electrochemical microcell technique, which uses extremely small capillaries (diameters between 5 and 10 μm). Experiments were conducted in 0.1 M NaClO₄ at 25 °C. The β-Mg₁₇(Al, Zn)₁₂ precipitates were found to have the highest corrosion resistance, whereas the eutectic phase was very active (pitting potential of approximately −1400 mV vs. Ag/AgCl). The α-Mg solid solution displayed better corrosion resistance than the eutectic phase.     

Stress corrosion cracking of cold-worked austenitic stainless steels

Stress corrosion cracking (SCC) of AISI 304L and AISI 316L stainless steels, cold-worked under various conditions (i.e. at different degrees of deformation obtained by drawing and rolling at room temperature and at liquid nitrogen temperature) has been carried out in H₂O containing 1000 ppm Cl^? at 250°C and in a boiling MgCl₂ solution. The effect of heat treatments at 400 and 900°C on the SCC of previously cold-worked steels has also been studied. Particular attention was directed towards heat treatment at 400°C. In steels deformed at room temperature, it increases the SCC resistance. By contrast, for steels deformed at liquid nitrogen temperature, heat treatment at 400°C reduces the SCC resistance if carried out for short periods of time (1–6 h). Hardness measurements, structural analyses via X-rays, scanning and transmission electron microscopy (SEM and TEM), as well as modified Strauss tests, seem to prove that reduced stress corrosion resistance is not to be related to the chromium-rich carbides precipitation which could have been accelerated by the presence of α′-martensite. Instead, they tend to suggest that perhaps this phenomenon is connected to an increase in the level of internal micro-stresses which are generated by a reciprocal re-ordering of the α′ and γ structural phases.     

Inhibition of steel corrosion by polyaniline coatings

Polyaniline (PANI) coatings were electrosynthesised on steel samples (13% and 4.44% Cr) using sulphuric and phosphoric acids as supporting electrolytes. Protective properties of PANI coatings in the supporting electrolytes were investigated by monitoring the open-circuit potential vs. time, and by applying electrochemical impedance spectroscopy. PANI layers have been found to provide corrosion protection. Thicker PANI layers at 530 mV vs. Ag/AgCl (3 mol dm⁻³ KCl) exhibit pure capacitive behaviour at low frequencies, and in addition a small resistance at high frequencies. Thinner layers at 530 mV exhibit a much higher resistance attributed to a higher degree of PANI-free electrode surface and/or to a significant amount of PANI transformed from emeraldine to leucoemeraldine form. The layer deposited in a phosphate solution appears to have better protective properties than the layer deposited in a sulphate solution. Therefore, PANI from phosphate solution was tested also in 0.1 mol dm⁻³ HCl. However, in the chloride-containing solution, the time of protection was significantly shorter.     

Einflüsse des Phosphors auf die Interkristalline Spannungsrißkorrosion von Kohlenstoffstählen

Influence of phosphorus on the intergranular stress corrosion cracking of carbon steels The effects of phosphorus on the intergranular stress corrosion cracking were studied for steels with 0.15% C and 0.4 or 2% Mn, the phosphorus contents were 0.003, 0.03 and 0.05% P. Constant strain rate tests were conducted at constant potentials in 55% Ca(NO₃)₂ at 75°C, in 5 N NH₄NO₃ at 75°C, and in 33% NaOH at 120°C. The strain rate was 10^?6/s. Different grain boundary concentrations of phosphorus were established by varying the annealing time at 500°C, they were determined by fracturing the samples in UHV and analyzing intergranular fracture faces by Auger-electron spectroscopy. In the nitrate solutions the toughness, i.e. resistance against stress corrosion cracking of the steels is somewhat decreased with increasing P-content within the range of the corrosion potential, that is ?300 mV_H to ?50 mV_H in NH₄NO₃ and ?80 mV_H to ?50 mV_H in Ca(NO₃)₂. It is shown that this effect is caused by phosphorus segregated at the grain boundaries. At potentials above ?50 mV_H the relative toughness is very low for all tested steels, also without stress intergranular attack is observed. In NaOH the steels are most sensitive against stress corrosion cracking at ?700 mV_H, here the phosphorus content or segregation shows no effect. At potentials varied in both directions the toughness increases and a somewhat negative effect of phosphorus becomes visible. In all tested electrolytes the effect of phosphorus is restricted to a small potential range. There is no effect of phosphorus in the range of the minimal toughness, thus the steels are sensitive anyway, with and without phosphorus segregation. Therefore low phosphorus contents in carbon steels cannot guarantee resistance against stress corrosion, most decisive are the potential and the electrolyte.     

Über den Einfluß der Strömungsgeschwindigkeit und der NaCl-Konzentration auf die Sauerstoffkorrosion unlegierten Stahls in Wässern

Influence of flow velocity and sodium chloride concentration on the oxygen corrosion of unalloyed steels in water Corrosion of mild steel in aerated water normally leads to pitting, and to an enrichment of anions within the pits. Even HCO₃^? ions can support localized corrosion. The intensity of this is remarkably increased by chloride ions. In equilibrium water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂ protective rust layers are formed after about 500 hs exposure not depending on flow rate and salt concentration. These layers decrease the mean corrosion rate to about 0.1 mm/a, but do not prevent localized corrosion. Protective passive layers with extremely low corrosion rates are formed in flowing water with Ca²⁺ and HCO₃^? ions and about 40 ppm free CO₂. The minimum flow rate can be estimated for a 3/4 inch pipe to lie between 0.35 and 1.5 m/s. Investigations with water containing 40 ppm free CO₂ without Ca²⁺ at pH 4.7 and with NaHCO₃ (pH 7.2) show at low pH high corrosion rates – as expected –, and at pH 7.2 formation of protective layers with poor reproducibility in comparison with the situation in equilibrium water. Thus, Ca²⁺ ions support the protective properties of rust layers. HCO₃^? ions are necessary for the formation of protective layers because these are not formed in pure NaCl solutions.     

Der Einfluß mechanischer Wechselbelastungen auf die Beständigkeit Mo-freier und Mo-haltiger Stähle gegen Spannungsrißkorrosion in Calciumnitrat-Lösung und Natronlauge

The influence of cyclic loading on stress corrosion cracking of Mo-free and Mo-bearing steels in solutions of calcium nitrate and sodium hydroxide The resistance of five unalloyed or low-alloy steels differing essentially in their Mo-contents, against SCC was investigated by electrochemically controlled SCC experiments in boiling 60 wt.-% Ca(NO₃)₂ solution (E_H = 0.15 V) and 35 wt.-% NaOH (E_H = -0.75 V). For characterizing the SCC response, threshold stresses O_G (criterion: crack depths 1 = 0) were evaluated with mechanical loading by alternating stresses (saw tooth characteristic, strain rates ε = 3.4. 10^?6 or 8.7. 10^?7s^?1, load cycle frequency 5. 10^?3 or 3. 10^?3Hz). If the threshold stresses obtained by constant load SCC experiments are near the YS or in the plastic range (Ca(NO₃)₂ solution), the threshold stresses are lowered considerably by cyclic loading. If, however, the threshold stresses obtained under constant load conditions are in the elastic range (NaOH), there is only little influence of cyclic loading on the o_G values.     

Effects of coolant chemistry on corrosion of 3003 aluminum alloy in automotive cooling system

In this work, effects of coolant chemistry, including concentrations of chloride ions and ethylene glycol and addition of various ions, on corrosion of 3003 Al alloy were investigated by electrochemical impedance spectroscopy measurements and scanning electron microscopy characterization. In chloride‐free, ethylene glycol–water solution, a layer of Al‐alcohol film is proposed to form on the electrode surface. With the increase of ethylene glycol concentration, more Al‐alcohol film is formed, resulting in the increase in film resistance and charge‐transfer resistance. In the presence of Cl^? ions, they would be involved in the film formation, decreasing the stability of the film. In 50% ethylene glycol–water solution, the threshold value of Cl^? concentration for pitting initiation is within the range of 100 ppm to 0.01 M. When the ethylene glycol concentration increases to 70%, the threshold Cl^? concentration for pitting is from 0.01 to 0.1 M. In 100% ethylene glycol, there is no pitting of 3003 Al alloy even at 0.1 M of Cl^?. Even a trace amount of impurity cation could affect significantly the corrosion behavior of 3003 Al alloy in ethylene glycol–water solution. Addition of Zn²⁺ is capable of increasing the corrosion resistance of Al alloy electrode, while Cu²⁺ ions containing in the solution would enhance corrosion, especially pitting corrosion, of Al alloy. The effect of Mg²⁺ on Al alloy corrosion is only slight.