Film Thickening on Nickel in Aqueous Solution in Relation to Anion Type and Concentration

Abstract

The variation of the open circutt potential of the Ni electrode in strongly aerated soluiion of different concentrations of Cl^?, Br^?, I^?, NO^?₃, SO^2?₄ and CrO^2?₄ is followdd till steady-stete values are established. In all solutions the steady potentials, E_sb, are approached from negative values, denoting film repair and thickening. Except in relatively concentrated Cl^? media, no Ni²⁺ is detected in solution. Plots of the Ni electrode potential, E, as a function of the logarithm of immersion time, t, are linear, satisfying the general relationship:

E = a_l + b₁log t ……(1)

where a₁ and b₁ are constants. Equation (1) is derived theoretically by taking into account the p-conducting properties of NiO, and assuming that the adsorption of the anions on the oxide film generates the electric field necessary to promoee the diffusion of Ni²⁺ through the oxide. The rates of oxide thickening in the presenee of different concentrations of the various anions are computed and discussed. Expressions are deduced relating E_st and the thicknsss of the oxide to the various parameters of the partial anodic and cathodcc reaction.The steady-stete potentials vary with the logarithm of the molar concentration of the various anions according to:E_st = a₂ ? b₂ log C ……(2)This relation is reached from a consideration of the structure of the double layer, and the effects produced thereon when the anions are specifically adsorbed on the surface of the electrod. Agreement between theory and experiment is established for Cl^?, I^? and SO^2?₄ soluiion. The adsorption of Br^? is assumdd to occur in the Br^2?₂ form. The results also indicaee that the CrO^2?₄ ion has a partial monovalent character. The adsorbability of the different anions tested decreases in the order: I^? (strong)> Cl^?> Br^?> SO^2?₄> NO^?₃ (weak)     

Effects of iron-containing phases on transformation of sulfur-bearing ions in sodium aluminate solution

Sulfides in the high-sulfur bauxite lead to serious steel equipment corrosion and alumina product degradation via the Bayer process, owing to the reactions of sulfur and iron-containing phases in the sodium aluminate solution. The effects of iron-containing phases on the transformation of sulfur-bearing ions (S^2–, S₂O₃^2?, SO₃^2? and SO₄^2?) in sodium aluminate solution were investigated. Fe, Fe₂O₃ and Fe₃O₄ barely react with SO₃^2? and SO₄^2?, but all of them, particularly Fe, can promote the conversion of S₂O₃^2? to SO₃^2? and S^2– in sodium aluminate solution. Fe can convert to Fe(OH)₃^? in solution at elevated temperatures, and further react with S^2– to form FeS₂, but Fe₂O₃ and Fe₃O₄ have little influence on the reaction behavior of S^2– in sodium aluminate solution. Increasing temperature, duration, dosage of Fe, mole ratio of Na₂O_k to Al₂O₃ and caustic soda concentration are beneficial to the transformation of S₂O₃^2? to SO₃^2? and S^2–. The results may contribute to the development of technologies for alleviating the equipment corrosion and reducing caustic consumption during the high-sulfur bauxite treatment by the Bayer process.     

The influence of sodium chloride on the atmospheric corrosion of steel

The influence of sodium chloride on the atmospheric corrosion of mild steel has been studied. Samples covered with sodium chloride crystals (8 m?g NaCl · cm^?2) have been exposed to an SO₂-free atmosphere at different relative humidities. The combined effect of sodium chloride crystals on a steel surface and sulphur dioxide in the atmosphere has been investigated at an SO₂-supply of 1 m?g SO₂ · cm^–2 · h^–1 (1 ppm SO₂). The corrosion attack was followed by periodic weighings. The corrosion products formed have been studied using the SEM-technique. On exposure of samples with sodium chloride in an SO₂-free atmosphere the extent of corrosion increased with increasing relative humidity from 58% to 90%, interrupted by a sharp minimum at about 87% relative humidity. The SEM-studies showed that tower shaped corrosion products were formed at a high relative humidity while filiform corrosion appeared when the relative humidity was lowered. At 90% relative humidity more corrosion was observed with clean steel samples and an SO₂-supply of 1 m?g ·cm^–2 ·h^–1 than with sodium chloride crystals on the surfaces (8 m?g NaCl ·cm^–2) in the absence of SO₂. In the combined influence of sodium chloride on the steel surfaces and sulphur dioxide in the atmosphere a synergic effect was noticed at 90% relative humidity. At 70% relative humidity no influence of an SO₂-supply of 1 m?g SO₂ ·cm^–2 ·h^–1 on the corrosion of steel samples with sodium chloride crystals on their surfaces could be observed.     

Pitting Corrosion Currents on Steel in Relation to the Concentration of the Inhibitive and Corrosive Anions under Natural Corrosion Conditions

Abstract

The changes with time in the pitting corrosion current density on a steel electrode with the concentration of both the inhibitive anions (CrO₄^2?, HPO₄^2? and WO₄^2?) and aggressive anions (Cl^?, Br^? and I^?) was followed using a simple electrolytic cell. In chromate solutions the pitting corrosion currents started to flow after an induction period, τ, which varied, in one and the same inhibitor solution, with the concentration of the aggressive anion, according to the expression: log τ =a?b log C_agg.

The pitting corrosion currents finally reached steady-state values which depended on the type and concentration of both the inhibitive and corrosive anions. At a constant inhibitor concentration,the corrosion current varied with the concentration of the aggressive anion according to: log i_corr = a_l + b₁ log C_agg, and with constant aggressive ion concentration according to: log i_corr =a₂ - b₂ log C_inh Comparison was made between the experimentally obtained values of a_l (a₂ and b₁ (b₂), and the corresponding computed values. The a₂ values show that the corrosivity of the three aggressive anions decreases in the order: Cl^? > Br^? > I^?; on the other hand the inhibitive efficiency of the inhibiting anions decreases in the order: CrO₄^2? > HPO₄^2? > WO₄^2?.     

Current efficiency of Ga electrodeposition under different anions concentrations

The gallium electrodeposition from alkaline solution has a very low current efficiency,the reason for which is still not quite understood.The effects of electrode materials used for gallium electrodeposition,as well as the effects of Na OH concentration and the anions concentrations in the solution,including SO_4~(2-),SiO_3~(2-),CO_3~(2-),AlO_2~-,F~-,and Cl~-,on the deposition were analyzed in this study.The suitable materials of SUS316–SUS316 were suggested for the gallium electrodeposition.Based on the electrode couples,the Na OH concentration of 4 mol L~(-1)for gallium electrodeposition exhibits the greatest current efficiency.Moreover,the current efficiency would decrease in the electrolyte along with the increasing concentration of the anions,except that,0.2 mol L~(-1)Cl-in the solution slightly improves the current efficiency of gallium electrodeposition.Moreover,the gallium deposited on the cathode from the solution with 0.6 mol L~(-1)SiO_3~(2-)appears tiny black in color and coarse.Meanwhile,SUS304 is shown to be not suitable to be used as cathode for the gallium electrodeposition from the alkaline solution.     

Stress—corrosion cracking of austenitic stainless steel in hydrochloric acid media at room temperature

Stress—corrosion cracking of solution quenched, type 304, stainless steel can occur at room temperature in HCl solutions ranging between 5·10^?1M and 1M HCl. The cracking observed in HCl solutions is similar to that previously observed in H₂SO₄ + NaCl and HClO₄ + NaCl solutions. Cracking occurs at ? 0·200 V (NHE), in the active potential region, it is under cathodic control, and it develops in conditions under which the corrosion rate of the external surface area is more or less constant and independent of the HCl concentration, in the range 10^?1 M?1 M HCl. At higher HCl concentrations, corrosion rates increase and uneven, general corrosion occurs instead of cracking. The development of pitting and stress—corrosion cracking under active conditions precludes the conclusion that active—passive cells always play a role in localized corrosion and, in particular, in stress—corrosion cracking. Under these conditions, it has been shown that sensitized and non-sensitized specimens behave similarly (giving rise in both cases to transgranular cracking); active—passive cells, due to chromium depletion at the grain boundaries, are not involved. Active—passive corrosion mechanisms can however arise at more noble potentials (0·100?0·200 V NHE), as in the case of HClH₂O₂ solutions of specific concentration, producing intergranular corrosion of the stainless steel in the sensitized condition.     

Identification and formation of green rust 2 as an atmospheric corrosion product of carbon steel in marine atmospheres

Green rust 2 (GR2(SO^2?₄ )) was detected amongst the products formed on a carbon steel rod exposed to atmospheric corrosion using X‐ray diffraction (XRD). The presence of green rust 2 has been related to sulphate reducing bacteria (SRB) in the sea spray. These were detected using test catch rods made out of inert material and posterior lab identification using Starkey culture. Likewise, after the exposure of said rods to sea environmental conditions, SRBs have been isolated from among the carbon steel corrosion products. The evolution of GR2(SO^2?₄ ) from GR1(SO^2?₄ ) was ruled out due to the tendency of this compund to produce GR2(SO^2?₄ ) in the presence of sulphate ions, as is the case here. Likewise, the evolution from GR1(Cl^?) has also been ruled out since in such case as this compound should be formed (it has not been detected in any of the 39 stations studied), the enormous affinity of the GRs with divalent anions (such as is the case with SO^2?₄ ) as opposed to the monovalent ions (such as is the case of the Cl^?) makes GR1(Cl^?) transform into GR2 (SO^2?₄ ).     

Mechanism of Action of Acidified Sodium Phosphate Solution as a Corrosion Inhibitor of Iron in Tap Water

Abstract

The influence of phosphate as acid Na₃PO₄ solution in tap water was investigated using gravimetric and potentiostatic techniques. It was found that the phosphates are adsorbed in the form of positively charged colloidal particles on the cathodic areas of the metal. The degree of coverage, θ, vs log concentration of PO₄^3? shows good agreement with Frumkin's isotherm in the form:

θ/1?θ exp (?2.55θ) = 2530.CpO^3?₄

The calculated standard free energy of adsorption was found to be ? G_ads° = ?28·8 kJ.mol^?1 (or ?6·9 kcal.mol^?1).     

Kinetics of the passivation of molybdenum in salt solutions as inferred from impedance and potential measurements

The oxide film formed on the molybdenum electrode in sodium salt solutions of different anions was found to thicken according to a logarithmic growth law and the electrode potential varied with time according to the equation E_h = α + β log t. The rate of oxide film thickening in the respective solutions was in the order Na₃PO₄ > Na₂SO₄ > NaCl > NaNO₃. The equilibrium potential was found to be independent of anion concentration in the case of Cl^- and NO^-₃ and was slightly dependent on the PO^3-₄ and SO^2-₄ concentrations. Complex plane analysis showed that the double-layer capacitance affects the measured capacitance and an electrical model was proposed to explain the alternating current behaviour of the electrode-electrolyte interface.     

Corrosion inhibition of steel by hydrazine

The corrosion inhibition of steel in a closed system containing hydrazine at room temperature has been established. The steady-state potential measurements as well as weight loss measurements agree in indicating that hydrazine was found to behave in a similar manner to anodic inhibitors. A sharp ennoblement of potential occurs at concentration of 10^?2M accompanied by complete protection. The mechanism of inhibition by hydrazine is assumed to be due to its preferential adsorption on the local anodes inducing anodic polarization. After a certain critical concentration of hydrazine is reached complete blocking of the pores occurs which causes complete inhibition. For hydrazine-sulphate or hydrazine-chloride mixtures, a linear relationship is found between log hydrazine concentration and log maximum tolerated concentration of the aggressive ion. For hydrazine-sulphide mixtures, a concentration of 5 × 10^?2M hydrazine could tolerate a wide range of concentrations of sulphide ions.     

Stress corrosion cracking behaviour of Al‐Li alloy 8090‐T8171 plate exposed to various synthetic environments

The stress corrosion cracking (SCC) behaviour of 8090‐T8171 plate material was investigated in short transverse direction performing constant load tests and constant extension rate tests under permanent immersion conditions. At an applied stress of 100 MPa, smooth round tensile specimens were exposed to synthetic environments containing chlorides and various nonhalide anions. Environment‐induced cracking was not observed in aqueous solutions of 0.6 M NaCl, LiCl, NH₄Cl, or MgCl₂. In 0.6 M NaCl solutions containing 0.06 M Na₂SO₄ or Na₃PO₄, the SCC behaviour of 8090‐T8171 plate was similar to that observed in pure 0.6 M NaCl solution. Sodium chloride solutions with additions of nitrate, hydrogen carbonate, or carbonate promoted stress corrosion cracking. Threshold stresses below 100 MPa were obtained from constant load tests using the latter environments. When sodium sulfite or sodium hydrogen phosphate was added, values being 100 MPa or slightly higher were determined. Lithium and ammonium present as cations in mixed salt electrolytes accelerated SCC failure. Lithium chloride solutions containing nitrate, hydrogen carbonate, carbonate, or sulfite were highly conducive to stress corrosion cracking. Very low SCC resistance was found for alloy 8090‐T8171 exposed to synthetic environments with additions of ammonium salts. Constant extension rate tests were carried out using notched tensile specimens. Displacement rates were in the range 2 × 10^?6 ? 2 × 10^?5 mms^?1. Aqueous 0.6 M NaCl solutions with additions of 0.06 M NH₄HCO₃, (NH₄)₂SO₄, or Li₂CO₃ promoted environment‐induced cracking with 8090‐T8171 plate, as indicated by severe degradation of notch strength. The constant extension rate testing technique did not indicate SCC susceptibility using sodium chloride solutions containing sodium sulfate or lithium sulfate. For specimens exposed to substitute ocean water a slight degradation of notch strength was found at the lowest displacement rate applied.     

Laboratory Testing of the Atmospheric Corrosion of Steel

The atmospheric corrosion of a mild steel (0.09 C) has been studied in flowing atmospheres containing 1, 10 and 100ppm SO₂ at 80, 85, 90 and 96% relative humidity. The flow rate was varied to give SO₂-supplies ranging between 0.4 and 250 μg SO₂ cm^?2 h^?1. After an exposure period of 300 h testing was continued up to 5000 h in atmospheres free from SO₂ at 85–100% relative humidity. Some of the samples were sprayed with distilled water to simulate wetting by rain or condensation. The corrosion attacks were followed by successive weighings. The composition of the corrosion products was studied using X-ray diffraction, IR-spectrometry and ESCA-technique. The flow rates of the SO₂-containing atmospheres were found to have a marked influence on the corrosion attack. The corrosion rates could not therefore be related to the SO₂ concentration of the atmosphere alone but rather to the supply of SO₂ per unit surface area and time. Exposures performed at high SO₂-supply resulted in formation of corrosion products more protective than those formed at lower SO₂-supply. ESCA-analysis proved formation of suphide containing corrosion products at SO₂-supply. ESCA-analysis proved formation of sulphide containing corrosion products at SO₂-supplies ≥ 8 μg SO₂ cm^?2 h^?1. Periodic spraying of the samples with distilled water were found to cause a drastic increase in corrosion attack.     

Structural and morphological characteristics of nanocrystalline copper electrodeposits from acid sulphate electrolytes

Copper is the presently favoured and future interconnect material in high-end microprocessors and memory devices because of its low electrical resistivity and higher electromigration than aluminium. The present investigation deals with the electrodeposition of nanocrystalline copper onto brass metallic foil from electrolytes containing copper sulphate (CuSO₄·5H₂O) as the source of metal ion and sulphuric acid (H₂SO₄). Benzotriazole (0.5?g?L^??1) and sodium lauryl sulphate (0.1?g?L^??1) were used as additives. The electrolyte was mechanically agitated and the temperature was maintained at 3°C?±?2°C. These additives have been found to be effective in reducing the grain size, grain boundaries and improving surface morphology of the copper films. They also improve the throwing power of the deposition electrolytes and hardness of deposits. X-ray diffraction (XRD) patterns obtained for the electrodeposited copper films showed polycrystalline cubic structure. The crystal size of the copper films was calculated by both XRD and atomic force microscopy (AFM) analysis. A uniform and pore free surface morphology was observed under SEM, and AFM investigation revealed the grain refining brought about by the additives.     

Electrochemical Investigation of the Nucleation and Propagation of Pits in Iron-Chromium Alloys

Abstract

Electrochemical measurements performed on single crystals and polycrystalline samples of Fe 13 Cr and Fe 16 Cr alloys in Na₂SO₄ + NaCl solutions differing in Ci^? ion concentration, have shown that during pitting the anodic current I increases with time t:

I～t^b Depending on experimental conditions, b may vary from 2 to 6. At b = 2 all the pits nucleate simultaneously, so that their number does not change with time of anodic polarisation. At b > 2 the number of pits increases with time.

Galvanostatic polarisation of polycrystalline samples in solutions containing much fewer Cl_? ions than SO₄^2? ions does not lead to a steady potential. Under these conditions periodic oscillations of potential occur, several hundred millivolts in amplitude. This phenomenon is probably due to the inhibiting action of SO₄^2? ions on the nucleation of pits by Cl^? ions, and it occurs only at certain concentration ratios of these two ions in the electrolyte. Inhibition of pit nucleation is also revealed during potentio-static and potentiokinetic measurements at potential values several hundred millivolts more positive than the breakdown potential measured galvanostatically.

An interpretation of above phenomena is given for both single crystals and polycrystalline alloy samples.     

The atmospheric corrosion of unprotected carbon steel – a comparison between field study and laboratory test

A comparison is made between the atmospheric corrosion of unprotected carbon steel in outdoor exposures and results from laboratory tests. The atmospheric corrosion in the outdoor exposures is expressed as a function of certain atmospheric parameters (e.g. SO₂ concentration). In the laboratory tests the atmosphere consisted of air containing known amounts of SO₂ and water vapour. The experiments were carried out in such a way that both supply and deposition of SO₂ was known. There was good agreement between the empirical formula and the results from the laboratory tests provided that the supply of SO₂ in the laboratory test was below ca. 0.5 μg · h^?1 · CM^?2.     

Influence of hydrostatic pressure on pitting of aluminium in sea water

Abstract

The effect of hydrostatic pressure on the corrosion of aluminium in sea water oj pH 8·2 containing 7ppm dissolved oxygen has been investigated at 20°C. On increasing the pressure from 1 to 300 atm, the corrosion rate increased, an effect attributable to an increase in the rate of the anodic reaction while the cathodic rate remained unchanged. The observed increased susceptibility to pitting corrosion with increasing pressure was found to correlate with the presence of increasing quantities of SO^2?₄ and Cl^? ions in the oxidation layer. The introduction of electron acceptors such as SO^2?₄ and Cl^? into n type compounds such as aluminium oxides increases the density of lattice dejects and promotes breakdown oj the barrier provided by the oxidation layer.     

Dissolution Current and Pitting Potential of Zinc in KOH Solutions in Relation to the Concentration of Aggressive Ions

Abstract

Potentiodynamic anodic polarisation curves for zinc were obtained in dilute (10^?3 M) solutions of KOH that favoured metal dissolution with only a slight tendency to passivate the zinc, and in more concentrated (10^?2 M) solutions that favoured passivation of the zinc. Addition of aggressive halide ions (Cl^?, Br^?, I^?) to the 10^?3 M solution caused a shift of the dissolution potential to more negative values and a marked increase in corrosion rate, suggesting adsorption of the anions on to the bare metal surface. The difference between the dissolution current density, i_o, in 10^?3 M KOH alone, and in the presence of halide ions, i_agg, was given by: log (i_agg?i_o) = a + b log C_agg, where a and b are constants.

In 10^?2 M KOH, small additions (2 × 10^?4 M) of aggressive ions did not affect the dissolution kinetics. At higher concentrations the c.d. increased sharply at a well defined potential, or pitting potential, E_p, given by the relation: E_p = α ? β log C_agg, α and β being constants. It is assumed that in 10^?2 M KOH the aggressive anion acts by penetrating the oxide, or by becoming incorporated in it as an impurity, altering its physical and electrical properties.     

Corrosion inhibition of Ti‐6Al‐4V alloy in sulfuric and hydrochloric acid solutions using inorganic passivators

The effect of iodate (IO₃^?), metavanadata (VO₃^?) and molybdate (MoO₄^2?) anions on the passivation of Ti‐6Al‐4V alloy in sulfuric and hydrochloric acid solutions was studied using open‐circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS). The alloy surface was examined by scanning electron microscopy (SEM). It was found that an optimum concentration of the passivator is essential for the corrosion inhibition process. Above this concentration the rate of alloy corrosion decreases as the concentration of the passivating ion increases. Scanning electron micrographs have shown that the flawed regions present in the alloy surface were repaired in the presence of the passivator anion. The optimum concentration of each anion and its corrosion inhibition efficiency for titanium and Ti‐6Al‐4V alloy have been determined. It turned out that the corrosion inhibition efficiencies of IO₃^?, VO₃^? and MoO₄^2? anions for the corrosion of Ti and Ti‐6Al‐4V in both hydrochloric and sulphuric acid solutions exceed 98%.     

Oxidation of polyacetylene in aqueous solutions containing sulphuric acid and selected supporting oxidants

It has been found that the addition of strong oxidants such as HNO₃, NaNO₂, KMnO₄, K₂Cr₂O₇ and NaIO₃ to an aqueous solution of sulphuric acid greatly facilitates the insertion of HSO₄^? ions into (CH)_x. [CH-(HSO₄)_y]_x formation occyrs even in less concentrated solutions in which sulphuric acid alone does not oxidize polyacetylene. Based on the above observation, a new analytical method for NO₃^? determination has been proposed. The method consists of acidification of any neutral, aqueous solution of a nitrate salt with sulphuric acid to 5.7 M. Although H₂SO₄ of such concentration does not oxideze polyacetylene, in the presence of NO₃^?, doping occurs due to the formation of free HNO₃ upon acidification. The doping reaction results in a significant increase of the electronic conductivity of the polyacetylene, σ. Log σ vs NO₃^? concentration plots are linear over a nitrate concentration range of 2 - 40 mM and can be utilized in chemical analysis. The selectivity of the proposed method is limited. Although the majority of common ions (HSO₄^?, Cl^?, PO₄^3?, etc. …) do not influence the measurements, the presence of other (than HNO₃) oxidants in the solution must be avoided.