Salt Immersion Research Articles

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ⩾ AZ91D-DC ⩾ AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.

Al Arc Spray Coating on AZ31 Mg Alloy and Its Corrosion Behavior

Aluminum arc spray process was employed to form an aluminum coating on AZ31 magnesium alloy substrate. Anti-corrosion property of the Al-coated specimens was evaluated by salt immersion tests and electrochemical tests. There was no apparent inter-layer diffusion or compound layer formation between the aluminum coating and the substrate under as-sprayed condition. To improve the density of the coating and the bonding strength between the coating and the substrate, a heat treatment process was conducted after spraying to form an interlayer by metallurgic diffusion. The interlayer consisted of large primary crystal Mg17Al12 phase and minute lamella structure that consisted of Mg17Al12 and α-Mg. The hardness of both the aluminum coating and the interlayer are higher than that of the substrate. In electrochemical experiments, the corrosion current density decreased from 2.4×10-1mA/cm2 of AZ31 to 7.1×10-3mA/cm2 of heat-treated specimen. It suggests that the corrosion resistance of AZ31 can be greatly improved by aluminum arc spray with a following heat treatment process.

Improved corrosion resistance of AZ91D magnesium alloy by an aluminium-alloyed coating

An aluminum-alloyed coating was applied onto the surface of magnesium alloy AZ91D. The coating formed in aluminium powder at 420 °C is rich in the β (Mg17Al12) phase. Polarisation curve, AC impedance, salt immersion and salt spray were carried out to investigate the corrosion behaviour and assess the corrosion performance of the coated magnesium alloy. It was found that a coated AZ91D specimen was much more corrosion resistant and harder than an uncoated one. The improved corrosion resistance was mainly ascribed to the high volume fraction of β phase in the coating.

Surface Modification on Magnesium Alloys by Coating with Magnesium Fluorides

A new technique has been developed for improving corrosion resistance on magnesium alloys. Specimens of AZ31 magnesium alloy were dipped into molten salt of NaBF4 at 723 K for various times, and then cooled, rinsed with water, and dried in air. Corrosion resistance in the surface treated specimens was evaluated by salt immersion test using 1 % NaCl solution as a time for occurring filiform corrosion. On an un-treated AZ31 alloy, the time for starting the filiform corrosion was about 1.2 ks, while on the surface treated specimen, the time was prolonged into about 1300 ks. Moreover, the surface treated specimen showed corrosion resistance in low pH solutions, such as 1 % HNO3 and HCl solutions.

塩化物環境におけるＭｇ‐Ａｌ‐Ｚｎ系合金の耐食性に及ぼすＡｌ含有量の影響

Influence of Al content on corrosion resistance of Mg-Al-Zn alloys containing 1 to 9 mass%Al was investigated in chloride environments. The corrosion rate decreased with increasing the Al content in the alloys regardless of the testing methods such as salt immersion test (SIT) and salt spray test (SST). The immersion tests in buffered solutions at constant pH also clarified that the effect of the Al content on the corrosion resistance is largest around pH 10, and diminished when pH is raised up to 12, where Mg-Al-Zn alloys are passivated by the formation of Mg-hydroxide on the surface. XPS analysis revealed that Al was concentrated in the surface film, especially around pH 10, and its concentration was increased with the Al content in the alloys. The high corrosion resistance of Mg-Al-Zn alloys with high Al can be attributed to the formation of the Al-rich surface film in chloride environments.

Adhesion of Electrochemically Formed Conducting Polymer Coatings on Al-2024

Polyaniline, polypyrrole, poly(N-ethyl aniline), poly(O-anisidine) and their composite coatings were electrodeposited on Al-2024 alloy. The electrodeposited films were characterised using IR spectroscopy. The peel test (ASTM 3359-97), the lap joint test (ASTM D1002) and electrochemical impedance spectroscopy were used to evaluate the adhesion of these coatings and the delamination area under the coating. It was found that the adhesion of these coatings depends on the electrodeposition parameters and the structure of the polymer film. There was no significant delamination of the coating after immersion in Harrison solution for 55 days. Contact angle analysis was conducted to evaluate the hydrophobicity of the film. Salt immersion and dc polarisation studies were used to evaluate the durability and the corrosion protection offered by the coatings, respectively.

マグネシウム合金ＡＺ３１と亜鉛めっき鋼板の腐食に及ぼす環境因子の影響

Influence of environmental factors on corrosion behavior of AZ31 magnesium alloy rolled sheet was investigated and compared with zinc coated steel sheet by three corrosion test methods, such as salt immersion test (SIT), salt spray test (SST) and combined corrosion cycle test (CCT). Corrosion rate of AZ31 in SIT was increased with NaCl concentration and decreased with pH rising. Zinc coated steel sheet, however, showed little dependence of NaCl concentration and pH. The corrosion rate of AZ31 in three corrosion test methods decreased in the order of SIT>SST>CCT. On the other hand the zinc coated steel sheet showed the reverse tendency. Electrochemical measurements in chloride solution showed that cathodic reactions of AZ31 and zinc coated steel sheet were reductions of water and dissolved oxygen, respectively. It was also clarified that the corrosion of AZ31 was controlled by anodic reaction, while the corrosion of the zinc coated steel sheet was dominated by diffusion of dissolved oxygen. The reverse tendency of the corrosion rates of AZ31 and zinc coated steel sheet in the three test methods can be explained by the fact that the pH rising in the vicinity of the metal surface is accelerated in the order of SIT, SST and CCT, and the diffusion of dissolved oxygen to the metal surface is increased in the same order.

Producing Fluorescent Marks on Atlantic Salmon Fin Rays and Scales with Calcein via Osmotic Induction

Abstract Calcified tissues of Atlantic salmon in both larval and postscale developmental life stages became labeled with the fluorochrome dye known as calcein in 7 min by sequential immersion in salt and calcein solutions. I introduce the term “osmotic induction” to describe this process. At 47 d posttreatment, fish that had been exposed to osmotic induction of calcein as nonfeeding larvae exhibited mortality and growth equivalent to that of nonexposed fish (P < 0.05). Atlantic salmon parr (at 5 months posthatch) exposed to osmotic induction of calcein exhibited no mortality during a 120-d posttreatment period. In addition to inducing a calcein mark on fin rays of all larvae and parr, the procedure also produced a distinct calcein mark on scales of parr. A second exposure of parr to osmotic induction 90 d after the first produced a conspicuous 2-band pattern on scales which corresponded to the two separate marking events. Calcein marks are invisible to the unaided eye but were observed without sacrificing...

Effect of divalent cations in low zinc ambient temperature phosphating bath

A new zinc phosphating bath, which produces coatings at relatively lower temperatures within a reasonable time by using of chemical accelerators has been devised. Improvement of the bath performance by the addition of divalent cations like calcium, manganese and magnesium has been studied. Bath formulation and operating conditions have been optimized by coating weight determinations. Corrosion resistance property of the resultant coatings has been evaluated in 1,000 ppm Cl− by electrochemical methods such as potentiodynamic polarization and impedance measurements. Results of the electrochemical techniques have been complemented by salt spray, humidity and immersion tests. Porosity and roughness of the coatings have also been studied. Results show that the phosphating bath with manganese addition gives good coatings within 30 min. Studies have shown that the corrosion resistance of the resultant coatings are much superior than the conventional coatings.

Corrosion and Mechanical Properties of Recycled 5083 Aluminum Alloy by Solid State Recycling

Corrosion and Mechanical properties of a recycled 5083 aluminum alloy by solid state recycling have been compared with those of a virgin extrusion which was processed from the ingot block. In the solid state recycling, the machined chips were extruded at 723 K with an extrusion ratio of 44 : 1 in air. As a result of the salt immersion tests, the mass loss of the solid recycled specimen was not less than twice of that of the virgin extruded specimen. The deterioration in corrosion properties for the solid recycled specimen was attributed to the excessive contamination of iron which promoted galvanic corrosion. As a result of tensile tests, the solid recycled specimen exhibited a good combination of high strength and high elongation to failure at room temperature. The excellent mechanical properties for the solid recycled specimen were attributed to the refined microstructure. However, the elongation to failure of the solid recycled specimen at elevated temperatures more than 573 K was lower than that of the virgin extruded specimen. The contamination of oxide particles is likely to be responsible for the lower elongation in the solid recycled specimen.

High-purity Magnesium Coating on Magnesium Alloys by Vapor Deposition Technique for Improving Corrosion Resistance

Microstructures in coated magnesium alloy with high purity magnesium fabricated by applying a vacuum deposition technique were investigated. Moreover, relationships between microstructures in coated and un-coated magnesium alloys and corrosion behaviors were interpreted by in-situ laser microscopic observations during salt immersion tests. Magnesium with 3N-grade and AZ31 magnesium alloy were used for an evaporation source and a substrate for deposition. Temperature of the substrates was changed resulting in change in temperature profile in a furnace in order to optimize deposition coating conditions for obtaining homogeneous microstructures and thickness in deposited layer. The coated specimen revealed superior corrosion resistance to those on 3N–Mg, AZ31 and AZ91E alloys, and comparable to that on 6N– Mg in salt immersion tests using 3% NaCl solution at 300 K for 587 ks. In-situ observations showed that inhomogeneity in microstructures, such as second phases and grain boundary segregations, deteriorate corrosion resistance in magnesium alloys. Therefore, pure magnesium coated layer without inhomogeneity in metallographic and electrochemical meanings can improve the corrosion resistance on magnesium alloys.

Corrosion Resistance and Adhesion Properties of the Chromium-Free Coating Using an Urethane-Base Resin for AZ91E Magnesium Alloy

Corrosion resistance and adhesion properties of a chromium–free (poison-free) coating using an urethane-base resin for AZ91E Magnesium alloy have been investigated by salt immersion tests and thermal and humid resistance tests. As a result of the salt immersion tests, the corrosion resistance of the chromium-free coated specimen was almost equal to that of the Chromium-based chemical conversion. Also, the thermal and humid resistance tests showed that no changes were found in the coating with an urethane resin base layer; however, exfoliation was observed in the coating with a phenolic resin base layer. In the coating with an urethane resin base layer, the boundaries between the base layer and the aluminum middle layer were relatively flat and there were no pores, though a lot of pores were observed between the base layer and the aluminum middle layer in the coating with a phenolic resin base layer. Therefore, it is suggested that the good adhesion properties of the chromium-free coating are related to the strong adhesion between the urethane resin base layer and the aluminum middle layer.

Electrochemical Studies on High-Temperature Corrosion of Silicon-Iron Coatings and Iron Aluminide Intermetallic Alloys by Molten Salts

Abstract The performance of Fe-Si coatings and an iron aluminide (FeAl) intermetallic alloy (FeAl40at%+0.1at%B+10vol%Al2O3) in molten salts containing vanadium pentoxide (V2O5) and sodium sulfate (Na2SO4) is reported. Corrosion and fouling by ash deposits containing V2O5 and Na2SO4 are typical corrosion problems in fuel oil-fired electric power units. High-temperature corrosion tests were performed using both electrochemical polarization and immersion techniques. The temperature interval of this study was 600°C to 900°C, and the molten salts were 80wt%V2O5-20wt%Na2SO4. Curves of corrosion current density vs temperature obtained by the potentiodynamic studies are reported, as well as the weight loss vs temperature curves from molten salt immersion tests. Both Fe-Si coatings and FeAl40at%+0.1at%B+10vol%Al2O3 showed good behavior against molten salt corrosion. The final results show the potential of these coatings and alloys to solve the high-temperature corrosion in fuel oil-fired electric power units.

Formation of Magnesium Films on Magnesium Alloys by Vapor Deposition Technique

In order to improve corrosion resistance of magnesium alloys, it has been attempted to coat the surface of the magnesium alloys with pure magnesium by vapor deposition technique. Commercial 3N-Mg and AZ91D alloy were evaporated at 1050 K in a stainless steel tube evacuated at about 10 -2 Pa, purified by a retort effect, and deposited on the surfaces of a substrate which was a magnesium alloy. AZ31. 1. It is considered that isolated small particles of magnesium are firstly formed on the substrate, and then they grow having faceted planes to cover the surface of the substrate. The coated and uncoated specimens of AZ31 ailoy were examined with salt immersion tests in 1% NaCI solution, which showed superior corrosion resistance of coated specimen to uncoated one.

The corrosion protection afforded by rare earth conversion coatings applied to magnesium

The corrosion protection afforded by cerium, lanthanum and praseodymium conversion coatings formed on pure magnesium and a magnesium alloy, WE43, has been studied using dc polarisation and ac impedance techniques. The coatings, which were formed by immersion in rare-earth salt containing solutions, reduced significantly the dissolution of magnesium in a pH 8.5 buffer solution. With continued immersion of the treated electrodes in the aggressive pH 8.5 solution, the coatings first appeared to become more protective, but after periods exceeding 60 min began to deteriorate. This is attributed to the formation of magnesium hydroxy corrosion products and mixed rare earth/magnesium oxide/hydroxide coatings, which on continued immersion became consumed by the formation of magnesium corrosion products.

99/02075 XPS analysis of carbon-supported platinum electrodes and characterization of CO oxidation on PEM fuel cell anodes by electrochemical half cell methods: Rheaume, J. M. et al. J. of Power Sources, 1998, 76, (1), 60–68

The corrosion behaviour of nickel and copper coatings in the current-collector area of separators and a Ni/Al intermetallic compound coating in the wet-seal area of separators for molten carbonate fuel cells is evaluated under immersion in molten carbonate salt. The corrosion-protection effect of nickel and copper coatings on 316L stainless steel is evaluated in an electrochemical half-cell by means of a potentiostatic polarization technique. In addition, the corrosion-protection behaviour of a nickel-coated layer in the anode current-collector area and a Ni/Al-coated layer in the wet-seal area of the separator are studied during operation of a single cell for 10 days. Nickel suffers no observable corrosive attack for up to 500 h in the anode gas atmosphere (H2/CO2). By contrast, nickel and copper exhibit less corrosion resistance than 316L stainless-steel during exposure to the cathode gas atmosphere (CO2/O2). Under a potentiostatic anodic polarization of −1.0 V (versus CO2/O2/Au), nickel-coated 316L stainless-steel displays a lower anodic current density than copper-coated and uncoated 316L stainless-steel. NiAl with a high melting point has better corrosion resistance than other Ni/Al intermetallic compounds. From single cell tests, it is found that nickel and NiAl coatings are efficient for the protection of stainless-steel in the anode area and wet-seal area, respectively.

A study of a corrosion-resistant coating for a separator for a molten carbonate fuel cell

Abstract The corrosion behaviour of nickel and copper coatings in the current-collector area of separators and a Ni/Al intermetallic compound coating in the wet-seal area of separators for molten carbonate fuel cells is evaluated under immersion in molten carbonate salt. The corrosion-protection effect of nickel and copper coatings on 316L stainless steel is evaluated in an electrochemical half-cell by means of a potentiostatic polarization technique. In addition, the corrosion-protection behaviour of a nickel-coated layer in the anode current-collector area and a Ni/Al-coated layer in the wet-seal area of the separator are studied during operation of a single cell for 10 days. Nickel suffers no observable corrosive attack for up to 500 h in the anode gas atmosphere (H2/CO2). By contrast, nickel and copper exhibit less corrosion resistance than 316L stainless-steel during exposure to the cathode gas atmosphere (CO2/O2). Under a potentiostatic anodic polarization of −1.0 V (versus CO2/O2/Au), nickel-coated 316L stainless-steel displays a lower anodic current density than copper-coated and uncoated 316L stainless-steel. NiAl with a high melting point has better corrosion resistance than other Ni/Al intermetallic compounds. From single cell tests, it is found that nickel and NiAl coatings are efficient for the protection of stainless-steel in the anode area and wet-seal area, respectively.

Role of Alloy Matrix and Dispersoid on Corrosion Behavior of Cast Aluminum Alloy Composites

Observations made during the corrosion of cast aluminum alloys dispersed with graphite/silicon carbide (SiC) particles were analyzed. Corrosion studies were carried out in 3 wt% sodium chloride (NaCl) solution using salt immersion and potentiodynamic techniques. Corresponding base alloys were subjected to identical test conditions to understand the influence of the dispersoid on corrosion behavior. In all cases, the composites exhibited higher corrosion loss over the respective matrix alloy. Weight loss increased with exposure time. The dispersoid/matrix and the precipitate/matrix interfaces were found to be favorable sites for nucleation of corrosion pits. Results were explained with the help of microstructural observations and electrochemical parameters.

Erosion and corrosion characteristics of an aluminium alloy-alumina fibre composite

The present paper describes the erosion-corrosion behaviour of an aluminium alloy (BS: LM 5) and its composite containing 10 vol.% alumina fibres. Tests were performed in 3% NaCl solution with and without sand particles (10 wt.%) using the rotating sample method. Salt immersion and potentiodynamic polarization studies of the specimens were also carried out in 3% NaCl solution. Both the tests showed increased weight loss with test duration. The composite suffered from more material loss than the base alloy. The erosion-corrosion test revealed a significantly higher extent of weight loss in the specimens as compared to the immersion test in an identical electrolyte (3% NaCl). Corrosion of the samples proceeded by the breaking-off of the oxide layer followed by nucleation of micropits and crater formation. Further, the composite suffered from additional attack by the electrolyte at the dispersoid-matrix interfaces. The potentiodynamic study indicated only a nominal shift in the corrosion potential of the composite, while a considerable increase in the corrosion current density of the base alloy was observed due to the incorporation of Al 2O 3 fibres. The presence of the alumina fibres in the matrix alloy increased the susceptibility of the composite towards pitting (at the fibre-matrix interface). In erosion-corrosion tests, weight loss of the samples was accelerated by the impingement of the electrolyte at corroded sites leading to easy removal of corrosion products from the surface. The presence of sand particles in the electrolyte further accelerated the process of material removal due to rapid formation of pits and craters and (partial) removal of alumina fibres from the surface. Four stages of erosion-corrosion, namely (1) incubation period, (2) accelerated erosion-corrosion, (3) deceleration period and (4) steady state material loss, were observed during the erosion-corrosion tests.

Preparation of Hg β′-alumina using reflux ion exchange

We have prepared partially exchanged Hg β′-alumina from single crystal and polycrystalline Na and Ag β′-alumina using the conventional molten salt immersion method and a newly developed reflux ion exchange method. In the reflux method, the sample is continually bathed in pure HgCl 2. The reflux method caused much higher Hg(II) exchange than the conventional immersion method. In single crystals of Ag β′-alumina, about 91 mole% of the Ag(I) was replaced by Hg(II) after seven days of reflux exchange compared to 15 m/o exchange with the conventional immersion method.