ABSTRACT One of the most difficult areas in which to replace chromate conversion coatings is in the aerospace industry. The difficulties relate primarily to the corrosion performance requirements (which are the strictest of any industry) combined with the widespread use of some of the most corrosion-prone of the aluminum alloys. This has slowed progress toward finding a single-step "drop-in" replacement for chromate.1 In the pursuit of less hazardous alternatives to chromate conversion coating processes, a number of transition metal oxyanions have been investigated,2 including permanganate. Bibber has published a range of studies related to the protection of Al using permanganate, including the incorporation of permanganate into boehmite (AlOOH) coatings,3 sealing of anodized coatings,4 and the generation of true conversion coatings.5-7 Although boehmite-type coatings have shown some promise in trials for aerospace applications,3 their high operating temperatures and relatively long processing times make them less attractive as chromate replacements.1,8 Permanganatebased conversion coatings require lower operating temperatures and shorter processing times,5-7 and despite the necessity to use a barrier seal to pass unpainted neutral salt spray (NSS) performance testing, these coatings show promise as potential replacements for chromate in the aerospace industry.1 The permanganate conversion coating is generated in a seven-step process, involving a solvent clean, alkaline clean, rinse, deoxidation, rinse, treatment in
ABSTRACT Antimony is finding use in semiconductor technology for making infrared detectors, thin film capacitors,1 and photoanodes.2 It greatly increases the hardness and mechanical strength of lead and so it is used as an alloying additive in lead alloys for grids in lead-acid batteries.3 The corrosion processes at open-circuit conditions were investigated in acid solutions by Laitinen, et al.4 The authors found that Sb dissolution at opencircuit conditions is coupled to oxygen reduction via the hydrogen peroxide (H2O2) route. At open-circuit conditions, Sb is covered with an oxide film formed by a field-controlled process.5 Anodic polarization increases the electric field across the film and leads to film growth by ionic conductivity.5 The potential of an Sb electrode is a corrosion potential and it is therefore of primary interest to know the nature of the different reactions that might occur at its surface. Studies of the chemistry of Sb emphasize mainly the formation of barrier-insulating oxide films in a range of electrolytes.5-9 Many investigators have focused their attention on the stability and the dissolution reactions of these oxide films. There is agreement that the anodic oxide films electroformed on the Sb electrode under potentiodynamic or galvanostatic conditions consist of an inner porous antimony oxide (Sb2O3) layer and an outer antimony pentoxide (Sb2O5) layer. The experimental findings showed that the nature and stability of the antimony oxide film are functions of different parameters like formation medium, dissolution medium, formation voltage, formation current density, and temperature.8-9 In a recent work, Pavlov, et al., used an ellipsometric technique to investigate the nature and stability of oxide films formed on Sb electrodes.10 The authors found that potentiodynamic oxidation of Sb takes place within two potential regions. They also observed a break in the chronopotentiometric (E-t) curves in the 3-to-4-voltage range during galvanostatic anodic oxide growth. The authors attributed these findings to anodic growth of
ABSTRACT High-level radioactive waste (HLRW) storage tanks, constructed at U.S. Department of Energy (DOE) sites to support the weapons industry, have been in service since the 1950's and will continue to be in service well in excess of their projected lifetimes.1 As the tanks age, corrosion mitigation is essential to minimize the risk of geological waste migration, as well as exposure to workers, the public, and the environment. The HLRW tanks of concern were fabricated from carbon steel and contain alkaline nitrate/chloride waste. Established more than 20 years ago, the first goal of the U.S. DOE HLRW mitigation program was to determine ideal tank chemistries to maintain the carbon steel in a passive state. This program has relied on both theory and laboratory experiments to set tank chemistry that specify acceptable ratios of hydroxide concentration, nitrate concentration, chloride concentration, and nitrite concentration (added as an inhibitor to control nitrate stress corrosion cracking as well as pitting corrosion).2-4 Deviations from ideal chemistries are known to occur, for example, when waste is transferred between tanks. Ideally, one would like to be able to determine, in due proximity, when these deviations begin to influence tank integrity. Of primary concern is pitting corrosion as pit sites not only act as leakage points, but it is believed that they act as precursors to stress corrosion cracking (SCC). One might envision several methods for evaluating changes in the corrosion electrochemistry of a tank, for example,
ABSTRACT Nowadays, steel corrosion has become one of the major causes of durability failure in existing concrete structures around the world. Although concrete is considered to be a durable construction material, it is, however, subject to deterioration caused by a number of factors that range from poor workmanship and material to environmental effects. It is said that under ideal conditions, the reinforcements that are embedded in concrete will not corrode. This is due to the high alkalinity of the concrete pore water solution, which ordinarily has a pH value of about 13, which provides a protective environment for the steel due to the formation of thin film of passivating iron oxide around its surface.1-2 This insoluble oxide film prevents oxygen from reaching the steel and inhibits corrosion.3 However, when this passive film is destroyed due to carbonation or chloride attack, the initiation of corrosion could be instigated. As the reinforcement corrodes, the volume of corrosion products formed may be as large as 600% of the original metal,4 thus causing internal pressures in concrete that lead to the cracking and spalling of the concrete cover. Today, one of the most common forms of deterioration observed in concrete structures is corrosion induced by carbonation. Carbonation is a phenomenon that occurs when carbon dioxide (CO2) gas in the air penetrates concrete and reacts with hydroxides to form carbonates. The ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to the reduction in the alkalinity of the pore solution
ABSTRACT The susceptibility to stress corrosion cracking (SCC) of Type 321 (UNS S32100) stainless steel (SS) under simulated petrochemical conditions containing thiosulfate and chloride (20 wt% sodium chloride [NaCl] + 0.01 M sodium thiosulfate [Na2S2O3], pH = 2) was assessed using the slow strain rate tensile (SSRT) test and static load (U-bend) tests at the free corrosion potentials. In the SSRT, effects of environmental 2 factors, such as chloride (Cl) plus thiosulfate (S2O3 ), Cl concentration, solution pH, and temperature, on the susceptibility to SCC of Type 321 SS were critically examined. In addition, factorial design experiments using Yates's algorithm quantitatively estimated the individual and interactive effects of temperature, Cl concentration, and solution pH on the SCC susceptibility of Type 321 SS. In the U-bend tests, specimens were immersed in an autoclave containing deaerated 20 wt% NaCl + 0.01 M Na2S2O3 aqueous solution (pH = 2) for 1,400 h at either 80°C or 300°C. Results of the SSRT tests indicated that the effects of environmental factors on the SCC susceptibility of Type 321 SS decreased in the following order: temperature effect >> solution pH effect > Cl concentration effect. The mechanism of SCC induced by corrosion pits or titanium carbide (TiC) particles (5 µm to 10 µm) is discussed. In addi
Corrosion has always been a problem in the petroleum refining and the petrochemical operations. The petrochemical process elements, such as furnace tubes, valves, and pipelines, frequently perform at high temperatures and in severely corrosive environments; therefore, heat- and corrosion-resistant alloys, e.g., austenitic stainless steels, have been used widely in the petrochemical industries because of their excellent mechanical strength and toughness. However, in chloride-containing, high-temperature environments, pitting, crevice corrosion, and stress corrosion cracking (SCC) are often associated with the operation.1-5 In addition, it was found that the factors most affecting corrosion of structural materials in the petrochemical industry are chloride (Cl) and hydrogen sulfide (H2S).6-10 H2S is an important constituent of refinery sour waters and is formed by the decomposition of organic sulfur compounds that are present at elevated temperatures.10-13 The Type 321 (UNS S32100)(1) stainless steel (SS) used in this study was a titaniumstabilized austenitic SS. Sensitization (the precipita-
ABSTRACT It has been reported that the pitting corrosion behavior of Alloy 600 (UNS N06600)(1) and some stainless steels in chloride solutions depends on the solution temperature.1-6 That is, pitting potentials decrease steeply with increasing temperature up to about 200°C, while at higher temperatures they steadily decrease or remain almost constant. Pit morphologies are also correlated with the temperature and change from isolated pits at low temperatures to a filiform or general corrosion type of attack at higher temperatures. Manning and Duquette4 suggested that a change in the semiconductivity of the oxide film was responsible for the effect of temperature on susceptibility to pitting. However, Carranza and Alvarez5 showed that passive films became more porous and hence less protective as the temperature increased, leading to a change in the pitting behavior. Stellwag6 proposed that a crystalline film was possibly formed at high temperature. However, he was unable to present direct experimental evidence. Although it is known that the temperature dependence of pitting of Alloy 600 in chloride solutions depends on the properties of the passive films, there is little information about the compositions and structures of these films as a function of temperature. In this study, x-ray photoelectron spectroscopy (XPS) and transmission electron diffraction were used to examine the composition and structure of the passive films formed on Alloy 600 in a deaerated 0.282-M
ABSTRACT The corrosion behavior of copper-brazed joints with a ternary filler metal, Cu-P-Ag (74.75% Cu, 7.25% P, 18% Ag) in the form of wire for brazing, was studied in 400 g/L, 700 g/L, 850 g/L, and commercial lithium bromide (LiBr) solution (850 g/L LiBr solution with 4.3 g/L lithium chromate [Li2CrO4] as inhibitor and 0.08 g/L lithium hydroxide [LiOH] as pH control) at 25°C. Corrosion resistance was studied on three different geometrical types of electrodes designed to study copper-brazed joints. Type 1 or filler metal (FM) was tested to study the influence of the alloying elements on the copper corrosion resistance. Type 2 or melted filler metal (MFM) was used to study the thermal effect on the filler metal corrosion behavior. Type 3 Cu//(CuP-Ag) or brazed copper was configured to study the brazing method, the melting process on copper, and the galvanic effect on copper-brazed joints. Corrosion resistance was estimated from the polarization curves. Metallographic examination was carried out to characterize the microstructure of the samples. The results showed that Li2CrO4 improved corrosion resistance of brazed joints with inhibiting efficiencies up to 95%. In the commercial solution, the alloying elements in copper improved corrosion resistance; however, the melting process shifted open-circuit potential (EOCP), critical potential (Ec), and corrosion potential (Ecorr) toward more negative values. Galvanic studies showed that copper behaved as the anode when coupled to the simulated copper-brazed joint, Type 3 configuration; however, it behaved like the cathode with the melted filler metal in the most concentrated solutions.
Aqueous solutions containing high concentrations of lithium bromide (LiBr) are used as the most effective absorbent in absorption heating and refrigerating systems that use natural gas or steam as energy sources1-5 because of their favorable thermophysical properties, high heat of hydration, high solubility of solid phases, good thermal stability, and appropriate viscosity.6 However, it can cause serious corrosion problems of structural materials in a heat absorption plant.7 The high electrical conductivity of copper is matched by its excellent thermal conductivity, which makes copper the first choice for the manufacturing of heat exchangers. Comparing copper, aluminum, and steel, copper is by far the best conductor of heat. In addition, copper and copper alloys present good mechanical properties and corrosion resistance in such environments. Copper tubes and fittings are easily joined metallurgically by soldering or brazing. Brazed joints with filler metals that melt above 450°C and capillary fittings are used for refrigeration piping where high joint strength is required or where service temperatures can be as high as 177°C.8 Copper-phosphorus brazing alloys (BCuP),9 often with silver added as a third element, are used extensively for joining copper, especially in refrigeration and air-conditioning copper piping and electrical con-
ABSTRACT A new approach was developed for achieving a low electrochemical corrosion potential (ECP) on structural materials exposed to high-temperature water, even without the addition of pure hydrogen. This approach uses metal hydride or alcohol acting as an alternate hydrogen source as well as providing an insulating oxide surface. Preliminary experimental data on various materials showed that the addition of these alternate reductants lowers ECP and enhances the catalytic bene?t of noble metal-treated surfaces.
INTRODUCTION During the operation of boiling water reactors (BWR) under normal water chemistry (NWC) conditions containing high oxygen concentrations, stress corrosion cracking (SCC) of structural materials, such as sensitized Type 304 (UNS S30400)(1) stainless steel (SS), is known to be a major materials performance problem. It has been reported that SCC susceptibility in BWR is in?uenced by the corrosion potential, which is controlled by the concentrations of oxidizing and reducing species. The electrochemical corrosion potential (ECP) is also affected by the electronic conductivity of oxide ? lms formed on metals in high-temperature water, since electron insulating ? lms on metal surfaces cause a low corrosion potential to be formed. By lowering the corrosion potential of SS to below a critical potential (< ?230 mV vs. standard hydrogen electrode [SHE]) the susceptibility to SCC is markedly reduced (Figure 1).1
The ECP is the mixed potential associated with the equilibrium of redox reactions occurring on a metal surface and the metal oxidation. In BWR water, cathodic currents associated with the reduction of oxygen (O2) and hydrogen peroxide (H2O2) are balanced by anodic currents involving hydrogen oxidation and corrosion of the metal. The corrosion current for Type 304 SS slowly decreases over months of exposure in a reactor water as the oxide thickness on the surface increases with time.
ABSTRACT During the kraft pulping process, wood chips are cooked in white liquor, which primarily contains sodium sulfide (Na2S) and sodium hydroxide (NaOH). In this process the lignin is fragmented into smaller segments whose sodium salts are soluble in the cooking liquor, leaving cellulose and hemicellulose in the form of intact fibers, which can be further processed for papermaking. The resultant liquor with dissolved organics after pulping is called black liquor (BL) as a result of its dark color. BL is concentrated and used in the kraft recovery boiler as a fuel to recover inorganic pulping chemicals and heat energy. During pulping, some hydrogen sulfide (H2S) and organo-sulfur compounds are formed that may result in the distinct smell of pulp mills. Oxidation of BL, an optional recovery stage, was first implemented in the paper industry as a means of reducing sulfur-based odors and the chemical makeup requirements for sulfur.1-2 Today, oxidation of BL is receiving new interest with the realization of decreased viscosity and lower fuel values of the BL. Lower fuel values allow mills to increase pulp production by consuming more liquor in existing recovery boilers, and still recovering necessary pulping chemicals. Changes in the general corrosion and stress corrosion cracking (SCC) susceptibility of used alloys or candidate alloys due to BL oxidation treatment are the main concern addressed by the present study. A key component of oxidized BL corrosivity is the extent to which the oxidation is carried out. Depending upon the extent of oxidation, sulfide in the BL may react to form NaOH and sodium thiosulfate (Na2S2O3), as shown in Equation (1), or complete oxidation may convert the sulfides into sulfates via the reactions in Equations (1) and (2). Oxidation of both sulfide and
ABSTRACT Corrosion behavior of stainless steel clad rebar (SCR) was investigated in both model solutions (initial pH: 10, 12.6, and 13.6, respectively, Cl up to 15%, ~5 M) and concrete (up to 8% admixed Cl by weight of cement). The unblemished acid-pickled Type 316L (UNS S31603) stainless steel (SS) lateral surface of SCR (in both solutions and concrete) was found to have high corrosion resistance comparable to that observed for solid SS bars in concrete. Limited evaluations conducted at 40°C showed some tendency for incipient corrosion of the sound lateral SCR surface under the most severe test conditions. Cut-end rebar termination by a proprietary SS cap attached with organic adhesive or a weld overlay showed no failures in limited yearlong experiments at room temperature. However, there was one cap adhesive failure when tested at 40°C, underscoring the need for extended evaluation of cut-end terminations. SCR specimens with small cladding breaks (1-mm or 4-mm drilled holes), as expected, developed corrosion of the exposed carbon steel (CS). However, corrosion of the CS was delayed or did not take place in some cases, possibly as a result of the stochastic nature of corrosion initiation and the small area of CS exposed at the breaks. Cladding separation at the CS/SS interface (delamination), likely aggravated by expansive corrosion product accumulation, was observed in some instances around the corroding CS at the break. There was indication that corrosion product accumulation at some of the breaks mitigated the corrosion progression.
Solid austenitic stainless steel (SS) rebar has shown very promising corrosion performance in chloride-contaminated concrete but at a cost much greater than that of conventional plain carbon steel (CS) rebar.1-6 Stainless steel clad rebar (SCR) with a carbon steel core offers the potential for performance comparable to that of solid SS rebar but at a moderate cost.7-8 Until very recently, most commercial SCR has been produced by the Stelax method, based on the NUOVINOX process:7,9 A SS pipe (typically 10 cm diameter with several mm wall thickness) filled with steel granules is used as the starting billet to make the finalized product by hot rolling. The final cladding is typically ~0.8 mm thick, normally metallurgically bonded to the CS substrate, which is sintered during hot rolling and develops a hypoeutectoid pearlitic microstructure similar to that of common carbon steel rebar. Figure 1 shows a typical scanning electron microscopy (SEM) appearance at the cladding interface and the corresponding elemental composition profile obtained by x-ray energy-dispersive spectroscopy (EDS). The line of precipitates ~5 µm away from the edge of the etched grain boundary region are likely chromium oxides as indicated by EDS spot analyses. Optical micrographs revealing the pearlitic microstructure of the CS are shown in the Results section. The product thus made is normally used with the mill scale removed by pickling. Production runs of tens of tons have been made by this process on several