Role of complexes in inhibition of mild steel by zinc–1-hydroxyethylidene-1, 1-diphosphonic acid mixtures

Abstract

The action of 1-hydrox yethylidene-1, 1-diphosphonic acid (HEDP) in combination with zinc ions as a corrosion inhibitor for mild steel in oxygen containing aqueous solutions has been investigated in relation to the presence of different zinc levels, the presence of aggressive anions, and different pH values. The highest inhibition effect was obtained at a 2·7:1 zinc/HEDP molar ratio, corresponding to 60 ppm zinc, and was associated with a significant anodic effect. In the absence of chloride and with no free forms of the inhibitor components, the zinc-HEDP mixture exhibited good protection over a wide range of pH (6·5-9·5). However, in a weakly acidic medium, protection could not be achieved. Calculations based on the dissociation constants of the HEDP and the stability constants of the possible zinc-HEDP complexes showed that this inhibition can be attributed to the presence of the 2:1 zinc-HEDP complex (Zn₂H_-1L^-) at a sufficient level. It is found that this anionic complex can behave as an anodic, passivating, inhibitor at a concentration as low as 0·00015M (20 ppm zinc-32 ppm HEDP mix ture). This passivity is prevented by the presence of sufficient chloride. The critical chloride concentration in these studies is approximately ten times higher than the complex concentration, in molar terms. It is also found that free phosphonate is aggressive and can prevent passivity due to the preferential formation of soluble iron-HEDP complex. The tolerance of the 2:1 complex to free phosphonate is substantially less than to chloride, approx imately equimolar.     

不锈钢在模拟混凝土孔隙液中的腐蚀行为研究

利用动电位扫描和交流阻抗法研究了普通碳钢与不锈钢在模拟混凝土孔隙溶液中的腐蚀行为及其差别,以及Cl-浓度、溶液pH值对腐蚀行为的影响,从而探讨了不锈钢取代碳钢作为混凝土钢筋骨架的可能性.结果表明：即便混凝土碳化导致孔隙液pH值下降后,氯离子对不锈钢的影响仍然较小,钝化膜致密完好,而普通碳钢的耐蚀性明显变差,钝化膜极易被氯离子侵蚀而破坏.     

Corrosion of maraging steels in chloride solutions

Abstract

The effect of a maraging heat treatment on the mechanical properties and corrosion behaviour of a special grade of heat treatable stainless steel has been studied. Although the mechanical properties such as strength and hardness of this steel are enhanced after the maraging heat treatment, its corrosion resistance is reduced as a result of the formation of intermetallic precipitates at the grain boundaries. In addition, oxide scales are formed on the surface of the specimens after the heat treatment owing to high temperature oxidation and these scales play an important role in the corrosion behaviour of the aged specimens. Corrosion studies have been conducted on underaged, peak aged, and overaged specimens with and without oxide scales and on specimens in the solution annealed condition. Impedance and potentiostatic anodic polarisation techniques in 3·5%NaCl of pH 1, 4, 7, and 10 at room temperature have been used. The corrosion performance order of this maraging steel with oxide scales in acidic, neutral and alkaline solutions is solution annealed> overaged> underaged> peak aged. The removal of oxide scales from the surface of the aged specimens by electropolishing considerably enhances the corrosion resistance without altering the performance order. The surface treated specimens behave in a similar manner to the solution annealed specimens in chloride solutions of pH 7 and 10. This tremendous increase in the corrosion resistance value is a result of the formation of a passive. film on the surface of the specimens during the surface treatment. In immersion tests the surface treated specimens show no signs of corrosion after immersion in 3·5%NaCl at room temperature for a prolonged period of time.     

Photoelectrochemical investigation of ac modulated passive films on 304 stainless steel in weak alkaline and neutral solutions

Abstract

A photoelectrochemical investigation has been carried out on an ac modulated passive film on AISI 304 stainless steel in 0·1M Na₂ B₄O₇ and 0·5M Na₂ SO₄ solutions. The results of the photocurrent measurements indicate that the photoresponse is determined by defects in the electronic structure of the films when the defects lead to localised states in the band gap region and that the photoresponse is sensitive to the passive condition and testing solutions. It is concluded that the passive film on 304 stainless steel is in a highly disordered amorphous state.     

Chloride induced pitting initiation on 304L stainless steel in acidic sodium sulphate solutions

Abstract

The kinetics of pit initiation on 304L austenitic stainless steel in aqueous sodium sulphate solutions containing various concentrations of sodium chloride have been investigated by induction time measurements using the potentiostatic test method. Oxide films were grown on the surface of the steel by potentiostatic oxidation in the passive region. Pitting induction time has been found to be dependent on the oxide film thickness: the thicker the oxide film, the longer the induction time. Measurements at temperatures rangingfrom 20 to 80°C gave an apparent activation energy for pitting initiation of ~ 48·1 kJ mol^?1, which suggests that the mechanism of single ion transport through the film is the rate determining step.     

Stainless steel reinforcing bars – reason for their high pitting corrosion resistance

In harsh chloride bearing environments stainless steel reinforcing bars offer excellent corrosion resistance and very long service life for concrete structures, but the high costs limit a more widespread use. Manganese bearing nickel‐free stainless steels could be a cost‐effective alternative. Whereas the corrosion behavior of stainless steels in alkaline solutions, mortar and concrete is quite well established, only little information on the reasons for the high pitting resistance are available. This work reports the results of pitting potential measurements in solutions simulating alkaline and carbonated concrete on black steel, stainless steel DIN 1.4301, duplex steel DIN 1.4462, and nickel‐free stainless steel DIN 1.4456. Duplex and nickel‐free stainless steels are fully resistant even in 4 M NaCl solutions with pH 13 or higher, the lower grade DIN 1.4301 shows a wide scatter between fully resistant and pitting potentials as low as +0.2 V SCE. In carbonated solutions with pH 9 the nickel‐free DIN 1.4456 shows pitting corrosion at chloride concentrations ≥3 M. This ranking of the pitting resistance can be rationalized based on XPS surface analysis results: both the increase of the Cr(III)oxy‐hydroxide and Mo(VI) contents in the passive film and a marked nickel enrichment beneath the film improve the pitting resistance. The duplex DIN 1.4462 shows the highest pitting resistance, which can be attributed to the very high Cr(III)oxy‐hydroxide, to a medium Mo(VI) content in the film and to a nickel enrichment beneath the film. Upon time, the protective properties of the surface film improve. This beneficial effect of ageing (transformation of the passive film to a less Fe²⁺ containing, more hydrated film) will lead to higher pitting potentials. It can be concluded that short‐term solution experiments give conservative results in terms of resistance to chloride‐induced corrosion in reinforced concrete structures.     

Feasibility of utilizing 2304 duplex stainless steel rebar in seawater concrete: Passivation and corrosion behavior of steel rebar in simulated concrete environments

The feasibility of using 2304 duplex stainless steel rebar in seawater concrete was determined by studying the passivation and corrosion behavior of steel in solutions simulated curing and service stage of concrete, respectively. The results demonstrate that 2304 duplex stainless steel rebar could be used with seawater concrete because of a stable passive film formed on the steel surface during the curing stage of concrete even in the presence of 2 M chloride ions. However, due to the synergistic effect of concrete carbonation, the rebar suffered a corrosive attack by chloride due to the lack of OH^? inhibition.     

Electrochemical behaviour of 254SMO stainless steel in comparison with 316L stainless steel and Hastelloy C276 in HCl media

Abstract

The electrochemical behaviour of 254SMO stainless steel in HCl solutions has been studied and compared with that observed for other alloys using open circuit potential measurements, potentiodynamic polarisation curves, and SEM analysis techniques. 254SMO does not display pitting corrosion at room temperature in HCl solutions having concentrations ≤5·0M. Its performance is comparable with that of Hastelloy C276 in HCl media in solutions of strength ≤1·0M. The critical pitting temperature for 254SMO exceeds 76°C in 3M HCl solutions.     

Polyaniline coating on 304 stainless steel by electropolymerisation from aqueous NaOH solution for corrosion protection in NaCl medium

Abstract

Polyaniline (PAn) coating was electrochemically synthesised on 304 stainless steel using cyclic voltammetry in 0·25M NaOH solution containing 0·1M aniline. Characterisation of the adhesive and stable PAn coating was carried out by cyclic voltammetry, Fourier transform infrared spectroscopy and scanning electron microscopy. The protective properties of PAn coating on 304 stainless steel were elucidated using linear anodic potentiodynamic polarisation and Tafel test in 3·5%NaCl solution. Linear anodic potentiodynamic polarisation test results proved that the PAn coating improved the degree of protection against pitting corrosion in NaCl solutions. Tafel test results showed that the PAn coating appeared to enhance protection for 304 stainless steel in 3·5%NaCl solution.     

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.     

Influence of Thiourea on the Dissolution of Mild Steel in Strong Hydrochloric Acid

Abstract

The dependence of the corrosion rate of a 0·06 % C steel upon the pH and chloride ion activity of solutions of hydrochloric acid at 30°–50° has been examined. A significant amount of simultaneous chemical dissolution takes place as the strength of acid is increased and this leads to anomalously high values of the Tafel parameters, and renders the Stern ‘polarisation resistance’ treatment inoperative. Additions of thiourea produce cathodic stimulation at low concentrations and general inhibition at high concentrations. The results are mterpreted in terms of thiourea dissociation to hydrosulphide ion.     

Inhibiting effect of octadecylamine on pitting corrosion behaviour of stainless steel type 1.4541 up to 250 °C

The inhibiting effect of octadecyclamine (ODA) on chloride induced localized corrosion of austenitic stainless steel type 1.4541 (AISI 321) is described at temperatures from 150 °C and 250 °C indeaerated aqueous solutions with chloride concentrations up to 10^{- 1} M. Results of electrochemical (current-potentiAl-curves, impedance spectroscopy) as well as corrosion tests on mechanical loaded samples (constant extension rate test), and surface analytical characterization (Auger electron spectroscopy) showed the inhibiting effect of ODA. The increase of the corrosion resistance of the investigated material is interpreted by an ODA-film on steel surface as a diffusion barrier, and the formation of a compact hot-water oxide layer, caused by the increase of pH resulted from ODA-cracking and dissociation.     

Corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments in the micropores of concrete in the propagation period

Steel embedded in concrete passivates due to the alkaline nature of the concrete pore solution. Reinforcement corrosion develops when the alkalinity is neutralized. Corrosion, in turn, induces acidification of the surrounding pore solution. In the present paper, corrosion rate of corrugated steel bars is studied and measured at different pH values in solutions simulating chloride environments. The media considered consists of saturated calcium hydroxide solutions containing Na and K⁺, neutralized with ferrous chloride. This latter substance is the soluble compound produced during the corrosion of steel after chloride attack. Hydrochloric acid solutions of different pHs were prepared in order to compare the steel corrosion rates in these solutions with those observed in ferrous chloride solutions of the same pH. A comparison of polarization resistance measurements (R_p) with gravimetrically weight loss determined is presented. Tafel slopes results are also included. Additionally, a comparison is made between measurements of AC impedance with those of the R_p method. The results indicate that the corrosion rate in the studied media follows the general trend found in other media of similar pH values: corrosion increases in acidic solutions, remains rather stable for pH range 3-11 and decreases significantly in highly alkaline solutions.     

Effect of nitrite in corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments of micropores of concrete in the propagation period

Inhibitors in concrete are usually used to be effective in alkaline or neutral solutions, however, when corroding by chlorides, there is a local acidification and therefore it is necessary to study the effect of the inhibitor in acid pH values. Measurements of the corrosion rate of corrugated steel bars have been carried out in solutions simulating electrolytic chloride environments in the micropores of concrete in the propagation period. It has been studied the effect of sodium nitrite as a corrosion inhibitor when added to the mentioned solutions. The solutions considered consisted of sodium chloride for neutral condition and ferrous chloride for acid condition. This is the soluble compound produced during the corrosion of steel as a result of chloride attack. Comparison of the results of polarization resistance with gravimetrically determined weight losses are presented. Also, results of electrochemical impedance spectroscopy are presented here. It has not been observed a significative improvement in using nitrite as inhibiting agent in these systems. The corrosion seems to be related to the [Cl⁻]/[OH⁻] ratio in three different regions of pH identified from acid to alkaline pH values.     

Corrosion and Corrosion Inhibition of Reinforcing Steel: I. Immersed in Alkaline Solutions

Abstract

The electrochemical behaviour of reinforcing steel in stagnant alkaline solutions, especially calcium hydroxide, has been investigated. It was found that the oxidation processes that take place on steel are determined by the degree of surface oxidation of the sample and by dissolved oxygen, but not by the type of cation present.

In aerated solution, ferrosoferric oxide (Fe₃O₄) is the intermediate oxidation product on the steel surface while ferrous hydroxide is the intermediate product in de-aerated solution.

In pure alkaline solutions, the critical pH above which stable passivity occurs is 11·5. This value increases when the alkaline solution is contaminated with aggressive ions or when reinforcing steel having mill-scale on its surface is used. A linear relationship holds between the pH of the alkaline solution and the highest concentration of sodium chloride that can be tolerated (CC1-): pH = n logCcc^? + K where nand K are constants.

Calcium hydroxide solutions prepared fronl water of different local sources act differently towards steel. Thus, in saturated calcium hydroxide solution prepared from either distilled or tap water, the steel becomes passive. In the corresponding solution in mineral or sea water, the metal corrodes. On the other hand, mineral water becomes inhibitive when the pH is increased to 13. Sea water could not be tolerated by mere increase in pH.     

Corrosion inhibition by tungstate in aqueous solutions and its application to coal–water slurries

Abstract

Corrosion rates of AISI 1010 steel both in the presence and absence of sodium tungstate as inhibitor were determined in chloride–sulphate solutions of concentrations typical of water used in coal plants. Corrosion rate data of AISI 1010 steel as a function of pH and sodium tungstate concentration showed that 100 ppm tungstate at pH 8·5 gave 87% inhibition. When associated with other chemical species in a binary or multicomponent system the inhibitor efficiency rose to 99%. Results obtained with sodium tungstate alone or in association with hydroxyethylidene diphosphonic acid (HEDP) and zinc sulphate are presented and discussed. The application of tungstate inhibitor in coal–water slurries has been explored.     

Umschlagspunkt or Flade Potential of Iron

Abstract

Flade potentials if iron and redox potentials of the Fe³⁺/Fe²⁺ system have been determined in de-aerated sulphate solutions, pH 3–5, and the effect of air on the Flade potential of iron in borate buffers, pH 6–9, has been examined.

The results extend the range of previous work and it is sho,wn that under anaerobic conditions over the pH range 3–5 the Flade potential is given by EF = 0·960 ? 0·158pH ? 0·064 log Fe²⁺whereas in the presence of air, in the pH range 7· 0?9·3, the normal Flade equation is obeyed.It is concluded that the Flade potential corresponds to the redox potential of the Fe³⁺/Fe²⁺ system, both in the presence and absence of air, the effect of oxygen being to modify the ferrous ion concentration.     

Galvanic cells encountered in the corrosion of steel reinforcement — II. Differential salt concentration cells

Two differential salt concentration cells have been studied at pH 6·5, 10, 12 and 12·5. The rest potentials of uncoupled steel immersed in chloride solutions of pH 6·5 and 10 were more negative than those obtained in pH 12 and 12·5. This is ascribed to the occurrence of general attack in the former and pitting in the latter solutions. The results of coupling showed that steel electrodes immersed in the half cells of the lower Cl^? ion concentration were completely protected from corrosion. The galvanic currents obtained from the differential salt concentration cells at pH 6·5 or 10 were considerably higher than those obtained at pH 12 or 12·5. In the anodes of the former cells general corrosion occurred while in those of the latter cells pitting corrosion occurred. The increase of the salt concentration range caused an increase in the galvanic current and in the cell e.m.f., negativation of the mixed potential and depolarization of the anodic and cathodic processes. The controlling factors of corrosion in the cells, at different pH values, were mainly determined by the kinetics of the anodic and to a lesser extent by the cathodic process.     

The corrosion of aluminium in solutions of sodium fluoride and sodium chloride

Abstract

The corrosion of 99·997% aluminium by 0·1 N-sodium fluoride and chloride has been examined in the presence of air over the pH range 2—13. When a solution of either salt was saturated with aluminium oxide, corrosion was arrested.

Under anaerobic conditions, 0·1 N-sodium fluoride attacked the air-formed oxide film and corrosion was rapid, in contrast no corrosion was observed in 0·1 N-sodium chloride solution.     

A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.