Neutral Salt Spray Exposure Research Articles

Preparing corrosion-resistant layered double hydroxide coating on magnesium alloy under mild condition

Hydrothermally prepared layered double hydroxide (LDH) coating is unfavorable to large-size magnesium alloy parts. Herein, PO43−- and OH−-intercalated MgAl-LDH coatings with good corrosion resistance and strong adhesion were first prepared under relatively mild conditions. The optimal coating displays an exceptionally low corrosion current density (5.68 nA cm−2), a high charge transfer resistance (831.3 kΩ cm2), and negligible morphological variation without visual corrosion after 15 days of neutral salt spray exposure. The superior long-term protection is attributed to the intelligent release of PO43− and formed phosphate depositions with extremely low solubility product constant to effectively self-repair the micro-defects.

Dual strategy of improved crosslinking degree and incorporated inhibitory fillers for high corrosion resistance sol-gel coatings

In this work, high corrosion resistance sol-gel coatings were prepared by the dual strategy of improved crosslinking degree and incorporated inhibitory fillers. Firstly, a special attention was paid to the mechanism of 2MI improving the crosslinking degree of coatings. Furthermore, the inhibitory fillers 8HQ@Ce-MOF were incorporated to endow coatings with active corrosion protection. Results showed that the 1.3 μm thick coating incorporated with 2MI and 8HQ@Ce-MOF appeared no corrosion pits after neutral salt spray exposure for 21 days, which was ascribed to 2MI improving the crosslinking degree, corrosion inhibition of guest molecule 8HQ, and corrosion suppression of host nanocontainer Ce-MOF.

Inhibiting Metal Galvanic and Carbon Corrosion in Aluminum Alloy-Carbon Fiber Reinforced Composite Joints by Spontaneous Deposition of Diazonium Adlayers on Exposed Carbon Fibers

The spontaneous formation of a diazonium molecular adlayer on the exposed fibers of a carbon fiber reinforced polymer (CFRP) composite was investigated as a surface treatment. The purpose of the surface treatment is to reduce the extent of galvanic corrosion on a trivalent chromium process (TCP) conversion-coated aluminum alloy when the two materials are mechanically joined as is common in aircraft structures. The adlayer functions to reduce the rate of dissolved oxygen reduction on the more noble carbon thereby reducing the rate of galvanic corrosion on the more active metal alloy. The formation of the adlayer (4-nitrophenyl (NP), 4-nitroazobenzene (NAB), or 2-fluorene diazonium tetrafluoroborate (FL)) was achieved by immersing the CFRP composite specimen in a solution of 5 mM diazonium salt dissolved in acetonitrile for a specified time (1–24 h) under open circuit conditions (so-called spontaneous deposition). Cyclic voltammetry (CFRP composite) and rotating disk voltammetry (glassy carbon) were used to investigate the oxygen reduction reaction kinetics in naturally aerated 0.5 M Na2SO4 (pH 5–6). The oxygen reduction reaction current was suppressed by greater than 90% by the NP and NAB adlayers and 80% by the FL adlayer. The surface treatment (24 h immersion) with NAB significantly reduced the galvanic corrosion damage on TCP conversion-coated aluminum alloy during a 14-day continuous neutral salt spray (NSS) exposure. The mass loss (mg/cm2) and corrosion intensity (g/m2y) were reduced by over 10×. The spontaneously formed NAB adlayers are stable during the salt-spray exposure and provide better inhibition of galvanic corrosion on the aluminum alloy and resistance to carbon corrosion than do NAB adlayers formed by an electrochemically assisted process reported on previously.

The Electrochemical Behavior of As-Prepared Aluminum Alloy A360 Produced by Selective Laser Melting Fabrication with and without a Trivalent Chromium Process Conversion Coating

We report herein on the electrochemical behavior and corrosion resistance of as-prepared aluminum alloy, A360 (AlSi10Mg), fabricated by additive manufacturing, with and without a trivalent chromium process conversion coating. Selective laser melting was the 3D printing process used for the alloy build. The corrosion suppression provided by the coating system was assessed through electrochemical measurements and accelerated degradation testing during a 14-day continuous neutral salt-spray exposure. The results indicate that the conversion coating can be formed by solution pretreating (degreasing and deoxidation) the alloy (X-Z plane perpendicular to the build plane) with its native surface roughness (as-prepared) followed by immersion for 10, 15 or 20 min in the coating bath. All three immersion times provide an equivalent level of corrosion protection to the alloy. The conversion coating suppresses both anodic and cathodic currents, increases the polarization resistance, and provides both anodic and cathodic corrosion protection to the as-prepared alloy. The conversion-coated specimens exhibit good stand-alone corrosion resistance during a 14-day neutral salt-spray exposure with corrosion intensity values (g/m2-year) ∼10x lower than values for the uncoated, as-prepared alloy specimens.

Investigation of the Trivalent Chromium Process Conversion Coating as a Sealant for Anodized AA2024-T3

The sealing of an anodic coating on AA2024-T3 with a trivalent chromium process coating (TCP-HF) was investigated. Specimens were anodized in 9.8 wt% H2SO4 at 15 V DC (23 min) producing an oxide coating weight of ca. 1200 mg ft−2 and a thickness of ca. 5 μm. Elemental analysis revealed the sealant forms across the outer oxide layer and within the pores up to a depth at least approaching 1 μm. Anodic and cathodic currents in potentiodynamic polarization curves were suppressed by 5–10× and low frequency impedance (Z0.01 Hz) values were 5× higher for anodized specimens sealed with TCP (∼5 × 106 ohm-cm2), as compared to the unsealed anodized controls. During continuous exposure to 3.5 wt% NaCl, Z0.01 Hz values remained unchanged and greater than 106 ohm-cm2 after 5 days for the TCP-sealed specimen and decreased by 10× for the unsealed anodized control after just three days. A 14-day (336 h) neutral salt-spray exposure produced no significant pitting, discoloration or major coating detachment on anodized specimens sealed with TCP. Overall, the results indicate that the TCP sealant increases the barrier properties of the anodic coating and functions to provide anodic and cathodic corrosion protection.

Evaluation of a Trivalent Chromium Process (TCP) Conversion Coating on AA2024-T3 That Requires No Surface Pretreatment

We report on the study of a new commercial trivalent chromium process (TCP) conversion coating, TCP-1, that requires no deoxidation or etching surface preparation of the aluminum alloy for optimum performance. The corrosion resistance provided by TCP-1 and a comparable conversion coating, TCP-2, both formed by immersion on AA2024-T3, were evaluated using scanning electron microscopy and energy dispersive X-ray analysis, digital microscopy, XPS depth profiling, electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Accelerated degradation testing was also performed to determine how the laboratory electrochemical data correlate with the coating's corrosion protection during neutral salt-spray exposure and full-immersion testing. The results indicate that TCP-1 produces a protective coating on the metal that minimizes corrosion to an equivalent degree as TCP-2; a conversion coating that requires a conventional deoxidation step prior to application. Electrochemical data revealed that both TCP coatings increase the polarization resistance by ∼100x and suppress anodic and cathodic currents by ∼10–100x, as compared to the uncoated alloy. Specimens coated with either TCP coating exhibited little degradation, discoloration or corrosion during a 14-day neutral salt-spray (ASTM B117) exposure and a 14-day full immersion in 3.5 wt.% NaCl at 55°C. In summary, the stand-alone corrosion protection provided by TCP-1 to this alloy is as good as the protection provided by a standard TCP coating.

Wear and Corrosion Properties of 316L-SiC Composite Coating Deposited by Cold Spray on Magnesium Alloy

In order to improve the wear and corrosion resistance of commonly used magnesium alloys, 316L stainless steel coating and 316L-SiC composite coating have been deposited directly on commercial AZ80 magnesium alloy using cold spraying technology (CS). The microstructure, hardness and bonding strength of as-sprayed coatings were studied. Their tribological properties sliding against Si3N4 and GCr15 steel under unlubricated conditions were evaluated by a ball-on-disk tribometer. Corrosion behaviors of coated samples were also evaluated and compared to that of uncoated magnesium alloy substrate in 3.5 wt.% NaCl solution by electrochemical measurements. Scanning electron microscopy was used to characterize the corresponding wear tracks and corroded surfaces to determine wear and corrosion mechanisms. The results showed that the as-sprayed coatings possessed higher microhardness and more excellent wear resistance than magnesium alloy substrate. Meanwhile, 316L and 316L-SiC coating also reduced the corrosion current density of magnesium alloy and the galvanic corrosion of the substrates was not observed after 200-h neutral salt spray exposure, which demonstrated that corrosion resistance of a magnesium alloy substrate could be greatly improved by cold-sprayed stainless steel-based coatings.

Electrochemical Evaluation of Corrosion Inhibiting Layers Formed in a Defect from Lithium-Leaching Organic Coatings

This work presents the electrochemical evaluation of protective layers generated in a coating defect from lithium-leaching organic coatings on AA2024-T3 aluminum alloys as a function of neutral salt spray exposure time. Electrochemical impedance spectroscopy was used to study the electrochemical properties on a macroscopic scale. An electrochemical model allowed to quantitatively link the electrochemical behavior with the physical model of the layer in the damaged area as studied by scanning electron microscopy. Local potentiodynamic polarization curves obtained from micro-cell measurements showed an increase of the passive range in the defect area due to the formation of a robust protective layer. Scanning vibrating electrode technique measurements confirmed the non-reversible long-term corrosion protection of these generated layers in the coating defect.

Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys

Lithium salts are being investigated as leachable corrosion inhibitor and potential replacement for hexavalent chromium in organic coatings. Model coatings loaded with lithium carbonate or lithium oxalate demonstrated active corrosion inhibition and the formation of a protective layer in a damaged area during neutral salt spray exposure. The present paper provides an abridged overview of the initial studies into this novel inhibitor technology for the active corrosion protection of aluminum alloys. Coating defects were investigated by microscopic techniques before and after exposure to corrosive conditions. Scanning electron microscopy analysis of cross-sections of the coating defect area demonstrated that the protective layer comprises a typical three-layered structure, which included a dense layer near the alloy surface, a porous middle layer, and a flake-shaped out layer. Potentiodynamic polarization measurements obtained with a microcapillary cell positioned in the coating defect area and electrochemical impedance spectroscopy confirmed the corrosion protective properties of these protective layers. The long-term corrosion inhibition of the lithium-based coating technology was tested in industrial coating systems.

Corrosion resistance and self-repairing behaviour of Cr(III) contained conversion coating on AA2024-T3

A Cr(III) contained conversion coating on AA2024-T3 was prepared in solution containing Cr2(SO4)3 and K2ZrF6. Its composition was analysed using X-ray photoelectron spectroscopy, and its corrosion resistance was characterised by electrochemical measurements and neutral salt spray exposure. To investigate its self-repairing effect, the coupling current between a small uncoated sample and a large coated sample was monitored through an artificial scratch cell. The coating mainly consists of zirconium oxide and chromium oxide and hydroxide; no Cr(VI) species is detected. It improves the corrosion resistance by hindering the anodic process as well as the cathodic process. Meanwhile, it possesses self-repairing effect, which is generated from the release of Cr(III) species from the coating, migration and then deposition on the nearby scratch area.

An investigation of the corrosion inhibitive layers generated from lithium oxalate‐containing organic coating on AA2024‐T3 aluminium alloy

The protective film formed in a defect by leaching of lithium oxalate from model organic coatings during neutral salt spray exposure has been investigated. A scribed area of about 1 mm width was introduced on the coated AA2024‐T3 aluminium alloy. The scribed area was examined before and after exposure in neutral salt spray environment for 4, 8, 24 and 168 h by scanning and transmission electron microscopies. It was found that the lithium oxalate was able to leach from the organic coating during neutral salt spray exposure and it promoted the formation of a film that provided effective corrosion protection to the alloy. The typical film morphology consists of three different layers, including a relatively compact layer near the alloy substrate, a porous middle layer and a columnar outer layer. Variation of the film morphology was also observed at different locations of the scribed alloy surface, which may be related to the difference of local concentration of lithium species. Electron energy loss spectroscopy detected lithium, aluminium and oxygen in the film. Although the film showed the varied morphologies in different regions of the scribed area, the alloy substrate was protected from corrosion when the film was formed. Copyright © 2016 John Wiley & Sons, Ltd.

Protective Film Formation on AA2024-T3 Aluminum Alloy by Leaching of Lithium Carbonate from an Organic Coating

An investigation into corrosion inhibition properties of a primer coating containing lithium carbonate as corrosion inhibitive pigment for AA2024 aluminum alloy was conducted. It was found that, during neutral salt spray exposure, a protective film of about 0.2 to 1.5 μm thickness formed within the area where an artificial defect was introduced by scribing through the coating to the base alloy. This film showed a multilayered structure consisting of a relatively compact layer near the alloy substrate, a porous middle layer and a columnar outer layer. The thicknesses of the layers varied, as a consequence of the difference in the local concentration of lithium species leaching from the primer coating. The presence of the film in the scribed area significantly reduced corrosion of the alloy, with little consumption of the metal substrate within the scribed area during neutral salt spray exposure. Furthermore, the presence of the protective film resulted in a significant increase of impedance modulus, measured after the salt spray exposure.

Effect of long-term neutral salt spray exposure on durability of adhesive-bonded Zr–Ti coated aluminum joint

In this study, the effect of long-term neutral salt spray (NSS) exposure (i.e., 50g/L concentration of salt solution at 25°C) on the retained strengths of Zr–Ti coated and bare lap-shear aluminum joints bonded with hem flange adhesive Henkel Terokal 8021 NB was investigated. A one-part toughened epoxide adhesive and Zr–Ti coated aluminum substrates (i.e., AA6014-T4 and AA6016-T4) were selected. Adhesive-bonded coated aluminum joints (ACJ) and bare aluminum joints (ABJ) were fabricated and exposed in NSS environment for various times. Quasi-static tests were conducted immediately following removal of the joints from the salt chamber at the ambient condition. It was found that while NSS exposure for 720h degraded little the strength of ACJ, it decreased the strength of ABJ. The Zr–Ti coating protected aluminum substrates from electrochemical reaction and consequently minimized the strength degradation. As the exposure time was prolonged to 1400h, the strength of ACJ was reduced drastically while the strength of ABJ was only degraded slightly. Fractography, differential scanning calorimetry, electrochemical potentiostatic polarization measurements, and X-ray photoelectron spectroscopy analyses revealed the strength degradation of ACJ was caused primarily by the corrosion of Zr–Ti coating. The passive film on bare aluminum provided a long time protection against NSS exposure, and consequently minimized the strength degradation for ABJ.

Evaluation of corrosion resistance and self-healing behavior of zirconium–cerium conversion coating developed on AA2024 alloy

In the present investigation, a zirconium–cerium conversion coating (ZrCeCC) was developed on aerospace aluminum alloy (AA2024) by simple immersion technique. Zirconium conversion coating (ZrCC) was also prepared for comparison. Surface morphology of ZrCC and ZrCeCC specimens exhibited smooth and cracked patterns respectively. Elemental analysis showed the presence of Zr and Ce in ZrCeCC specimen. XPS results indicated the presence of multiple oxidation states of Zr and Ce in the developed coatings. Potentiodynamic polarization results revealed the corrosion current density (icorr) values obtained for both coatings (between 0.6 and 0.85μA/cm2) were lower than bare specimen (1.2μA/cm2). However, ZrCeCC specimen exhibited the lowest icorr value of about 0.46μA/cm2 after 168h of immersion in 0.6M NaCl solution. EIS studies showed that higher coating resistance (Rcoat) value was obtained for ZrCeCC even after 168h of immersion. The results obtained from simulated scratch cell test revealed a continuous increase in charge transfer resistance (Rct) value for bare specimen which was exposed along with ZrCeCC in 0.1M NaCl solution. The self-healing behavior of ZrCeCC was confirmed from all electrochemical tests. After 168h of neutral salt spray exposure, the surface of ZrCeCC was almost comparable with the conventional chromate conversion coating.

The corrosion protection of AA2024-T3 aluminium alloy by leaching of lithium-containing salts from organic coatings.

Lithium carbonate and lithium oxalate were incorporated as leachable corrosion inhibitors in model organic coatings for the protection of AA2024-T3. The coated samples were artificially damaged with a scribe. It was found that the lithium-salts are able to leach from the organic coating and form a protective layer in the scribe on AA2024-T3 under neutral salt spray conditions. The present paper shows the first observation and analysis of these corrosion protective layers, generated from lithium-salt loaded organic coatings. The scribed areas were examined by scanning and transmission electron microscopy before and after neutral salt spray exposure (ASTM-B117). The protective layers typically consist of three different layered regions, including a relatively dense layer near the alloy substrate, a porous middle layer and a flake-shaped outer layer, with lithium uniformly distributed throughout all three layers. Scanning electron microscopy and white light interferometry surface roughness measurements demonstrate that the formation of the layer occurs rapidly and, therefore provides an effective inhibition mechanism. Based on the observation of this work, a mechanism is proposed for the formation of these protective layers.

Electrochemical and mechanical studies on influence of curing agents on performance of epoxy tank linings

Abstract Effect of different curing agents on the mechanical, chemical resistance and anticorrosive properties of tank linings based on phenolic + novolac epoxies has been studied. Proportion of two epoxies was chosen by monitoring the processing viscosity of the coating. Parts of hardener required for 100 g of the above resin (phr) was scrutinized using differential scanning calorimetry and flexibility measurements. As a result, five coatings were finalized and their mechanical properties were investigated by dynamic mechanical analyzer. Anticorrosive properties of these coatings were studied using electrochemical impedance measurements and neutral salt spray exposure test. Chemical resistance of these coatings was investigated by monitoring the weight change of the coating during immersion. Coatings cured with modified cycloaliphatic amine (amine value = 350), in spite of lower cross-link density, exhibited superior anticorrosive properties and reasonably good chemical resistance. Electrochemical impedance results correlated fairly well with actual solvent resistance property of the coating.

Service life prediction of organic coatings: electrochemical impedance spectroscopy vs actual service life

Electrochemical impedance spectroscopy (EIS) is used to derive an expression for predicting the service life of organic coating in a C4-type environment (industrial and costal areas with moderate salinity) as defined in ISO 12944 standard for paints and varnishes—corrosion protection of steel structures by protective paint systems. Three coating systems with a record of 2, 5, and 10 years of durability were selected for the study. The selection was also based on proven composition and dry film thickness (DFT) of the coatings as per ISO 12944. Electrochemical impedance measurements of the paint-coated panels were carried out by exposing the coated mild steel panels without scribe in different corrosive environments such as immersion in NaCl solution, neutral salt spray, etc. Neutral salt spray exposure was found to be the most severe corrosive environment among all the three coating systems. In most of the cases, EIS gave early indication of coating failure when compared to visual defects such as blistering and over-film corrosion.