Salt Immersion Test Research Articles

Studies on enhancement of surface mechanical properties of electrodeposited Ni–Co alloy coatings due to saccharin additive

NiCo alloy coatings were galvanostatically electrodeposited at room temperature from sulfate bath using saccharin (0 to 12gL−1) as an additive agent. These alloy coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), contact angle, microhardness, Electrochemical impedance spectroscopy (EIS), salt immersion test and tribology techniques. The XRD study confirmed the formation of Ni–Co alloy with a face centered cubic (FCC) structure. Depending on saccharin addition, the Co content varied from 27 to 7at.%. The microhardness and corrosion resistance showed enhancement due to saccharin addition. Ni–Co alloy with a 12at.% Co content obtained from 10gL−1 saccharin addition showed reduced grain size (12nm) and hydrophobic surface (113°) with the highest corrosion and wear resistant values as compared to others. Such coating on medium carbon steel fasteners showed significant enhancement in corrosion protection during salt immersion test.

Waterborne Epoxy Nanocoatings Modified by Nanoemulsions and Nanoparticles

Electrically conductive coatings are required for static charge dissipation in power ground network. In the present investigation electrically conductive nanocoatings were prepared by the incorporation of graphite, nano-SiO2 concentrate, acrylic nanoemulsion and fluorocarbon emulsion onto the waterborne epoxy polymer. The nanosize distribution of nano-SiO2 concentrate and nanoemulsion was characterized with laser diffraction analyzer and scanning electron microscopy (SEM). From the results of SEM image, the graphite particles were well distributed in conductive coating. The corrosion resistance and thermal stability of nanocoatings were comparatively studied by SEM and thermogravimetry (TG). The corrosion-inhibiting properties of the conductive nanocoatings were investigated by salt immersion test. The measurements of contact angle, bonding strength and heat-freeze charge demonstrated that 1.5–2.0 wt% nanoparticles improve the resistance to pollution, adherence and resistance to heat-freeze charge of conductive nanocoatings. The measured results of surface electric resistance of nanocoatings demonstrated that a small amount of nano-SiO2 particles could enhance the conductivity in the corrosive environment.

Effects of Conductive Pigments on the Anti-Corrosion Properties of Zinc-Rich Coatings

Effects of replacing partial zinc dust with conductive pigments on the anti-corrosion properties of ethyl silicate zinc-rich coatings with low zinc content were investigated. Salt spray test, salt immersion test and open circuit potential(OCP) measurement were utilized to evaluate anti-corrosion properties. Experimental results revealed that the effects of conductive pigments content on anti-corrosion properties are similar for di-iron phosphide powder, graphite powder and aluminum powder employed in this article. For every kind of conductive pigments, protection life in salt spray test, protection life in salt immersion test, galvanic protection life and initial OCP change with conductive pigments content increase in a similar way. They all reach an extreme point, and all the extreme points correspond to a same replacing ratio named best replacing ratio. The best replacing ratio is 20,15 and 15wt% for di-iron phosphide powder, graphite powder and aluminum powder respectively. Comparing with non-replacing coatings, conductive pigments-containing coatings at best replacing ratio possess both lower zinc content and better anti-corrosion properties.

Effect of NaVO<sub>3</sub> Concentration on Corrosion Resistance of Conversion Coating on AZ91D Magnesium Alloy

Anticorrosive oxide films were successfully prepared on AZ91D magnesium alloy by chemical conversion coating in La (NO3)3 electrolytes with and without NaVO3. The morphologies and chemical composition were characterized by optical microscope, scanning electron microscope (SEM) and energy dispersive analysis (EDX). The corrosion resistances of the oxide films were evaluated by salt immersion test. The result showed that the oxide film formed in electrolyte with NaVO3 is successful in providing superior corrosion resistance for AZ91D magnesium alloy.

Effect of microstructure on the zinc phosphate conversion coatings on magnesium alloy AZ91

The effect of the microstructure, particularly of β-Mg17Al12 phase, on the formation and growth of zinc phosphate conversion coatings on magnesium alloy AZ91 (AZ91) was studied. The zinc phosphate coatings were formed on AZ91 with different microstructures produced by heat treatment. The effect of the microstructure on the zinc phosphate coatings were examined using optical microscope (OM), X-ray diffraction (XRD), coatings weight and etching weight balances, scanning electron microscopy (SEM) and salt immersion test. Results showed that as-cast AZ91 contained a high volume fraction of the β-Mg17Al12 phase and it was dissolved into α-Mg phase during heat treatment at 400°C. The β-phase became center for hydrogen evolution during phosphating reaction (cathodic sites). The decreased volume fraction of the β-phase caused decreasing both coatings weight and etching weight of the phosphating process. However, it increased the crystal size of the coatings and improved corrosion resistance of AZ91 by immersing in 0.5M NaCl solution. Results also showed that the structure of the zinc phosphate conversion on AZ91 consisted of two layers: an outer crystal Zn3(PO4)2·4H2O (hopeite) and an inner which was mainly composed of MgZn2(PO4)2 and Mg3(PO4)2. A mechanism for the formation of two layers of the coatings was also proposed in this study.

Analysis of Local Corrosion Phenomenon on Interface between Mg-Al Alloy and Ti

Magnesium has poor corrosion resistance because of its less noble standard electrode potential (SEP). Therefore, it is easily corroded in first when contacting with other metals having a higher SEP. For alloy development of Mg matrix composites reinforced with Ti particles, the corrosion behavior between Mg matrix and Ti dispersoids is one of very crucial phenomena. In this study, the surface potential difference (SPD) between Mg-Al alloy (AZ80) and Ti was investigated by using Scanning Kelvin Probe Force Microscope, and the corrosion resistance was evaluated with that SPD. In this experiment, two specimens were prepared. One was a joined material of AZ80 and Ti by using spark plasma sintering (SPS) process in solid state (AZ80(solid)-Ti) and the other by melting AZ80 after SPS (AZ80(liquid) -Ti). The latter specimen included the reacted layer of enriched Al at interface between AZ80 and Ti due to diffusion of Al during melting. AZ80(solid)-Ti had no reacted layer at the interface. The measured SPD between AZ80 and Ti of AZ80(solid)-Ti was about 0.62V and mostly the same to that between Mg and Fe. The pair of Mg and Fe generally shows largely poor corrosion resistance. On the other hand, the SPD between AZ80 and the layer of enriched Al was about 0.29V and smaller than that between AZ80 and Ti. The results indicated the enrichment of Al at interface between AZ80 and Ti improved corrosion resistance of Mg-Al alloy matrix composites reinforced with Ti particles. AZ80(liquid)-Ti showed superior corrosion resistance to AZ80(solid)-Ti in salt immersion test.

Characterization of Composite Coatings Produced by Plasma Electrolytic Oxidation Plus Cathodic Electrophoretic Deposition on Magnesium Alloy

The composite coatings were prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) plus cathodic electrophoretic deposition (E-coat) to improve corrosion protection. SEM observation revealed that the organic layer was integrated with PEO film by physically interlocking; Potentiodynamic polarization(E-i) measurements in 3.5% NaCl solution, copper accelerated acetate salt spray(CASS) test and immersion test were employed to evaluate corrosion principle and corrosion rate of composite coatings. It is indicated that the corrosion resistance of composite coatings was improved obviously compared to the PEO film. There is no noticeable pitting corrosion on the composite coatings after 320h CASS test. The adhesion of composite coatings was evaluated by cross-cut and pull-off test. The results show that adhesion of composite coatings can reach to level 0 and 11.3 N/mm2 as a result of the blocking effect between organic layer and PEO film.

Chemical conversion coating on AZ31B magnesium alloy and its corrosion tendency

The morphology change of the magnesium matrix after pre-treatment and the morphology as well as the phase composition of chemical conversion coating formed by phosphate were studied using scanning electron microscope and X-ray diffraction. The corrosion resistance of the coating was studied by salt spray and damp test, and the corrosion tendency during salt immersion test was analyzed. The results show that the phase composition before and after pre-treatment is almost changeless, and the deep microflaw appears between α and β phases during acidic pickling. The phosphate conversion coating is mainly composed of Mg, MgO, and some amorphous phase, and it can provide a good protection for the AZ31B alloy. Results from corrosive morphology indicate that the growth and the corrosion resistance of the phosphate conversion coating are related to the forming process of the AZ31B matrix.

Relation between corrosion behavior and microstructure of Mg–Zn–Y alloys prepared by rapid solidification at various cooling rates

The relation between corrosion resistance and microstructure of Mg–Zn–Y alloys with a long period stacking ordered (LPSO) phase has been investigated. In order to clarify the influence of microstructure evolution by rapid solidification on the occurrence of localized corrosion such as filiform corrosion, several Mg 97.25Zn 0.75Y 2 (at. %) alloys with different cooling rates were fabricated by the gravity casting, copper mould injection casting and melt-spinning techniques and their corrosion behavior and microstructures were examined by the salt immersion tests, electrochemical measurements, XRD and TEM. When the cooling rate was less than 3 × 10 4 K s −1, filiform corrosion propagated in the early stage of salt immersion test, due to formation of a massive block-shaped LPSO phase during casting. On the other hand, when the cooling rate was increased up to 3 × 10 4 K s −1, rapidly solidified (RS) alloys exhibited excellent corrosion resistance because of grain refinement and formation of a supersaturated single-phase solid solution. Large-sized Mg 97.25Zn 0.75Y 2 alloys fabricated by consolidation of the RS ribbons also exhibited excellent corrosion resistance with passivity. Enhancement of microstructural and electrochemical homogeneities in the Mg–Zn–Y alloys by rapid solidification techniques results in the passivity of substrate materials.

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.

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.

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.

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.

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 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.

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 Rust-preventive Properties of Long-Chain Alkylphosphonic and Alkylphosphonous Acid Derivatives

Several organo-phosphorus derivatives were synthesized from fatty acids and alcohols and their rust-preventive properties were examined by three corrosion tests, such as salt immersion test, out-door exposure test and humidity cabinet test.As a result, the derivatives having free phosphonic or phosphonous acid group were hardly soluble in base oil, but when used together with sorbitanmonooleate, they dissolved in base oil and showed good rust-preventive properties. Especially, 9 (10) phosphonostearic acid, 9 (10) phosphonostearyl alcohol, 9 (10) - (hydroxyphosphinyl) stearic acid and 9 (10) - (hydroxyphosphinyl) stearyl alcohol were surpassing.