Salt Spray Conditions Research Articles

Comparative corrosion study of zinc phosphate and magnesium phosphate conversion coatings on AZ31 Mg alloy

In this study, zinc phosphate and magnesium phosphate conversion coatings were formed on AZ31 Mg alloy (AZ31) by immersion treatment in zinc phosphate and magnesium phosphate conversion coating (ZPCC and MPCC) solutions. The coatings were characterized by a scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The corrosion behavior and corrosion resistance of coatings were studied by immersion test, potentiodynamic polarization curves and salt-spray tests (SST) with and without paints coated by electrophoretic deposition. During the immersion test, corrosion initiated much slowly on the MPCC than ZPCC. The ZPCC-coated samples showed pitting corrosion while filiform corrosion was observed on the MPCC-coated samples. Although ZPCC showed thicker coating, more positive corrosion potential and lower corrosion current density than MPCC, but it was very susceptible to corrosion under salt-spray conditions comparing with MPCC. The electropainted MPCC also showed much longer stability under salt-spray conditions than electropainted ZPCC.

Salt spray and EIS studies on HDI microcapsule-based self-healing anticorrosive coatings

Anticorrosive property of hexamethylene diisocyanate microcapsule-based self-healing coatings was systematically investigated by salt spray and EIS measurements. The influences of microcapsule diameter, weight fraction and coating thickness on the anticorrosive performance of the scratched samples were studied under salt spray condition, which revealed the thicker coatings with larger microcapsules at 10wt.% demonstrated the best anticorrosion behavior. Additionally, the kinetics of self-healing process characterized by EIS measurement was parametrically analyzed in an equivalent circuit when the scratched coating was exposed to salt solution. A simplified model was established to explain the influences of these factors with consideration of scratch dimension.

The corrosion behaviors of Mg–7Gd–5Y–1Nd–0.5Zr alloy under (NH4)2SO4, NaCl and Ca(NO3)2 salts spray condition

The corrosion behaviors of Mg–7Gd–5Y–1Nd–0.5Zr alloys after T5 treatment under (NH 4 ) 2 SO 4 , NaCl and Ca(NO 3 ) 2 salt spray condition were investigated by weight loss rates, residual mechanical properties, scanning electron microscope (SEM), X-ray Diffraction (XRD) and potentiodynamic polarization tests. The corrosion degree of Mg–7Gd–5Y–1Nd–0.5Zr alloys in Ca(NO 3 ) 2 salt spray was very shallow by corrosion morphology and the corrosion route was extended along the surface in texture-like shape, while the alloy in NaCl and (NH 4 ) 2 SO 4 salt spray were major local corrosion and there were serious corrosion pits on the surface. The weight loss rates in (NH 4 ) 2 SO 4 , NaCl and Ca(NO 3 ) 2 salt spray was respectively 0.4147, 0.1618 and 0.0725 mg/(cm 2 d −1 ). The results of residual mechanical properties indicated that the corrosion order in salts spray of Mg–7Gd–5Y–1Nd–0.5Zr alloys is NH 4 SO 4 > NaCl > Ca(NO 3 ) 2 , which was consistent with the results of potentiodynamic polarization tests. The type of the salts will play a vital role in the initiation of the corrosion of EW75 alloy when they are used in the atmosphere environments. Inorganic salts with the smaller PH value after dissolution will have a stronger impact on the corrosion of EW75 magnesium alloys.

Experimental Investigation of the Corrosion Behavior of Friction Stir Welded AZ61A Magnesium Alloy Welds under Salt Spray Corrosion Test and Galvanic Corrosion Test Using Response Surface Methodology

Extruded Mg alloy plates of 6 mm thick of AZ61A grade were butt welded using advanced welding process and friction stir welding (FSW) processes. The specimens were exposed to salt spray conditions and immersion conditions to characterize their corrosion rates on the effect of pH value, chloride ion concentration, and corrosion time. In addition, an attempt was made to develop an empirical relationship to predict the corrosion rate of FSW welds in salt spray corrosion test and galvanic corrosion test using design of experiments. The corrosion morphology and the pit morphology were analyzed by optical microscopy, and the corrosion products were examined using scanning electron microscope and X-ray diffraction analysis. From this research work, it is found that, in both corrosion tests, the corrosion rate decreases with the increase in pH value, the decrease in chloride ion concentration, and a higher corrosion time. The results show the usage of the magnesium alloy for best environments and suitable applications from the aforementioned conditions. Also, it is found that AZ61A magnesium alloy welds possess low-corrosion rate and higher-corrosion resistance in the galvanic corrosion test than in the salt spray corrosion test.

Neutral salt spray tests on Fe–Al and Fe–Al–X

The corrosion resistance of Fe–Al and Fe–Al–X alloys under neutral salt spray conditions has been evaluated. For the binary alloys the corrosion resistance constantly improved up to an Al content of about 30 at.%. While higher Al contents did not better the corrosion resistance it may be further improved by addition of passivity-inducing elements. The phases that formed on the various alloys have been identified. Particularly small weight losses after neutral salt spray testing were observed for those alloys where thin and dense layers of Fe3O4 (magnetite) had formed on the surfaces during the tests.

SKP and FT-IR microscopy study of the paint corrosion de-adhesion from the surface of galvanized steel

Scanning Kelvin Probe (SKP) and FTIR microscopy were applied to study the atmospheric corrosion of galvanized steel coated by electrophoretic epoxy resin (ED) at a defect.The SKP was useful to determine the spatial separation of the electrochemical reactions at a defect and surrounding metal/paint interface and to evaluate the formation of the galvanic cells. FT-IR microscopy was helpful to identify the composition and distribution of the corrosion products in the galvanic cells.It was shown that the cathodic delamination of coating takes place after deposition of a thick water electrolyte film in the defect. The anodic undermining of the coating is favoured in case of atmospheric corrosion under thin electrolyte films. The anodic de-adhesion starting from defect reaching the zinc layer and from the non protected cut edge in case of exposure in the salt spray conditions was also determined.The role of the formation of confined volume underneath the delaminated paint on the rate of anodic undermining is discussed.

Response of the sexes of the subdioecious plant Honckenya peploides to nutrients under different salt spray conditions

AbstractSex dimorphic plants often show sex‐specific differences in growth and biomass allocation. These differences have been explained as a consequence of the different reproductive functions performed by the sexes. Such differences may determine the performance of each sex in different habitats and therefore might explain the spatial segregation of the sexes described in many dimorphic plants. We examined the growth, reproductive and physiological responses of the sexes of the subdioecious plant Honckenya peploides to two levels each of salt spray and nutrients, which are assumed to be important selective forces in coastal environments. We found sex‐related differences in H. peploides. In particular, females allocated proportionally more dry mass to reproduction and grew less and more slowly than males regardless of salt spray and nutrient conditions, which is interpreted as a trade‐off between reproductive and vegetative growth. Regarding physiological response, nutrients significantly increased values of photochemical reflectance index (PRI) in females but not in males, suggesting that photochemical efficiency is more limited by nutrients in females than in males. PRI values also suggest intersexual differences in protection requirements against photoinhibition. The study did not find sex‐differential responses to salt spray, which caused a decrease in reproductive effort in both sexes. The sex‐related differences in relative growth rates, reproductive allocation and photosynthetic features found here may contribute to explaining habitat‐related between‐sex differences in performance and, therefore, the spatial segregation of the sexes observed in H. peploides.

A Pseudoboehmite-Silane Hybrid Coating for Enhanced Corrosion Protection of AA2024-T3

This work reports the performance of pseudoboehmite (PB)-silane hybrid coatings for the corrosion protection of AA2024-T3. A survey of the coating pretreatment and treatment steps was undertaken to reveal the electrochemical impact of such treatments on AA2024-T3 and its constituents, with testing also done on model microstructural analogs including Al–4Cu, , and . The hybrid coating performs very well in immersion and salt-spray conditions. The protective effect of the PB pretreatment is not additive but synergistic. PB surface layers could be integrated as a precursor to subsequent coatings owing to the functionality they can impart to the surface.

Effect of Humidity and Thermal Cycling on Carbon-Epoxy Skin/Aramid Honeycomb Structure

Many modern military aircraft are constructed from composite and bonded structure, such as thin carbon-epoxy laminate bonded to Kevlar® and Nomex® honeycomb. Operation of these platforms in Australian and global conditions will subject the structure to potentially high levels of humidity, extremes in temperature, and for maritime operations, exposure to salt spray conditions. The thin composite laminate is likely to rapidly absorb moisture in a humid environment and enable permeation of moisture into the adhesive and core. In addition to the chemical influence of moisture on the composite structure, the moisture trapped in the honeycomb structure may freeze and expand with changes in altitude during operations or simply due to daily temperature fluctuations at the resident airbase. The combination of moisture ingress in the honeycomb structure and thermal cycling may lead to deteriorated strength of the honeycomb panels over time that would not be observed for long term humid exposure alone. Long term salt water absorption may also have an adverse effect on composites structures. This study investigates the effects of humid environments and thermal cycling on the mechanical properties of composite and honeycomb structures.

Fire and Corrosion Resistances of Intumescent Nano-coating Containing Nano-SiO 2 in Salt Spray Condition

The nano-concentrates and flame retardant nano-coating were prepared in thhis study. The effect of nano-SiO(2) on the corrosion resistance and fire resistance of ammonium polyphosphate-pentaerythritol-melamine (APP-PER-MEL) coating was investigated by differential thermal analysis (DTA), scanning electron microscopy (SEM), effective thermal conductivity (lambda/d), X-ray photoelectron spectroscopy (XPS) and fire protection test. The chemical action and endothermic effect of ammonium polyphosphate, pentaerythritol and melamine in traditional flame retardant coating were damaged by salt spray condition, whereas the flame-retardant additives in the nano-coating demonstrated the good chemical interaction in salt spray condition. The uniformly dispersed nano-SiO(2) particles could improve corrosion resistance of the coating, and hence nano-coating could remain the good fire-resistant properties even after 500 h salt spray test.

Effects of Variations in Salt-Spray Conditions on the Corrosion Mechanisms of an AE44 Magnesium Alloy

The understanding of how corrosion affects magnesium alloys is of utmost importance as the automotive and aerospace industries have become interested in the use of these lightweight alloys. However, the standardized salt-spray test does not produce adequate corrosion results when compared with field data, due to the lack of multiple exposure environments. This research explored four test combinations through three sets of cycles to determine how the corrosion mechanisms of pitting, intergranular corrosion, and general corrosion were affected by the environment. Of the four test combinations, Humidity-Drying was the least corrosive, while the most corrosive test condition was Salt Spray-Humidity-Drying. The differences in corrosivity of the test conditions are due to the various reactions needed to cause corrosion, including the presence of chloride ions to cause pit nucleation, the presence of humidity to cause galvanic corrosion, and the drying phase which trapped chloride ions beneath the corrosion by-products.

Investigation on dual corrosion performance of magnesium-rich primer for aluminum alloys under salt spray test (ASTM B117) and natural exposure

Magnesium-rich primers perform very well on outdoor exposure and actual test conditions, yet fail rapidly in accelerated corrosion testing (salt spray test – ASTM B117). To investigate the behavioral dichotomy, Mg-rich primers exposed to salt spray testing and natural weathering were characterized at periodic intervals. The results revealed the presence of a thin and porous magnesium hydroxide layer in primers exposed to salt spray, and in natural exposure, a thicker, protective magnesium carbonate layer was detected and characterized. Samples exposed to atmospheric carbon dioxide exhibit excellent corrosion resistance but salt spray conditions are not conducive to facilitate magnesium carbonate formation.

Corrosion resistance of phosphated Zn–Ni alloy electrodeposits

The corrosion behaviour of Zn–Ni (12%) alloy electrodeposits coated with three different crystalline or one amorphous phosphate films was examined under neutral salt spray conditions and by sample immersion in a 5% NaCl solution. The XRD and AFM techniques were applied for the chemical composition and morphology characterization, while electrochemical measurements were used for determination of the corrosion and porosity parameters. It has been established that application of phosphate films on the Zn–Ni electrodeposits surface extends the steel corrosion protection period from two- to threefold. The protective abilities of the amorphous film were detected to be significantly lower with respect to the crystalline one. The thickness and grain size of the crystalline phosphate films, as well as the corrosion product film composition, were determined not to be of the principle importance for the corrosion behaviour of Zn–Ni electrodeposits. The protective properties of the phosphate coatings were related principally to their porosity.

The influence of surface microchemistry in protective film formation on multi-phase magnesium alloys

The high strength:weight ratio of magnesium alloys makes them an ideal metal for automotive and aerospace applications where weight reduction is of significant concern. Unfortunately, magnesium alloys are highly susceptible to corrosion particularly in salt-spray conditions. This has limited their use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. The simplest way to avoid corrosion is to coat the magnesium-based substrate by a process such as electroless plating, which is a low-cost, non line of sight process. Magnesium is classified as a difficult to plate metal due to its high reactivity. This means that in the presence of air magnesium very quickly forms a passive oxide layer that must be removed prior to plating. Furthermore, high aluminium content alloys are especially difficult to plate due to the formation of intermetallic species at the grain boundaries, resulting in a non-uniform surface potential across the substrate and thereby further complicating the plating process. The objective of this study is to understand how the magnesium alloy microstructure influences the surface chemistry of the alloy during both pretreatment and immersion copper coating of the substrate. A combination of scanning electron microscopy, energy dispersive spectroscopy and scanning Auger microscopy has been used to study the surface chemistry at the various stages of the coating process. Our results indicate that the surface chemistry of the alloy is different on the aluminum rich β phase of the material compared to the magnesium matrix which leads to preferential deposition of the metal on the aluminum rich phase of the alloy.

Surface Modification of AZ91 Magnesium Alloy

Mg is the lightest structural metal with the highest specific strength. It is therefore considered as a solution for vehicular mass reduction so as to reduce the fuel consumption and greenhouse gas emission. Unfortunately, Mg and its alloys are highly susceptible to corrosion, particularly in salt-spray conditions. This has limited its use in the automotive and aerospace industries, where exposure to harsh service conditions is unavoidable. To improve its corrosion resistance it is viable to apply a desirable engineering coating on the Mg surface.This communication discusses the practical aspects of magnesium surface modification based on our review. A novel immersion coating process is also discussed along with the corrosion behavior of coated and uncoated AZ91 magnesium alloy.

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an “alkalisation”, “passivation”, “poisoning” effect or “shortcut” effect.

Corrosion Behaviour of Zn Coatings Electrodeposited from Alkaline and Acid Solutions

SUMMARYThe corrosion behaviour of Zn coatings deposited from acid and alkaline solutions was studied under the neutral salt spray conditions. XPS, EIS and voltammetric measurements were applied for the characterisation of Zn coatings after corrosion testing, while AFM was used for surface morphology examination. Lower corrosion rates were determined for the coatings deposited from the alkaline non-cyanide solution. Composition of the corrosion product film was determined to be the same for both types of coatings, therefore the differences in their corrosion behaviour can be related to the structural properties of the inner oxide layer of the corrosion product film. A thinner, but more compact oxide film on the coatings, deposited from the alkaline bath, possessed passivating characteristics, contrary to the coatings deposited from the acid electrolyte.

An epoxy novolac emulsion system for ambient cure protective coatings

Water-borne two-pack coatings based on a novel low particle size epoxy novolac emulsion, crosslinked with polycycloaliphatic polyamine provide a good balance of several properties. Because the films cured possess a high crosslink density of cross linkage, expectations such as rapid through film drying time, high hardness development at an early stage during cure, very good film formation at 10°C and 80% R.H., and chemical resistance can be met. However, there are also adverse aspects to consider regarding the combination of a water-insoluble hydrophobic polyamine and a modified epoxy novolac emulsion. eg no visible end of pot life, relative short pot life and a brittleness of coatings associated with a tendency to peeling when exposed to salt spray and humid conditions. The study also revealed that the quality of a cured two-component epoxy binder emulsion system depends to a great extent on the quality of the emulsion, particularly on the size and distribution of the droplets. A low particle size epoxy emulsion improves the film formation, drying time, water resistance, gloss, adhesion and hardness. For this purpose epoxy emulsions with a narrow particle size distribution and relatively low particle size (mean value ca. 0.4 µm) can be developed using a proprietary mechanical emulsion process. The aqueous coating system described may be useful for applications where conventional epoxy novolacs are already used as low temperature heat cure coatings like drill pipe coatings, oil pipelines and fuel storage tanks and in fields where their chemical, water and solvent resistance can be utilised. Further ambient cure applications may be protective coatings for indoor use, eg food factories, schools, hospitals and office buildings, industrial maintenance coatings for concrete, mortar, bricks, plaster and wood.

The Effect of Salt Spray Condition on the Corrosion Behavior of Spring Steels

To understand the influences of corrosion test conditions on the corrosion behavior of suspension spring steels, we investigated the corrosion loss, shape of pits and structure of rusts at the conditions of wet-ratio 8%, 33%, 100% for SUP7 steel and UHS2000, the τmax=1300MPa-grade suspension spring steel. It was revealed that the corrosion was the most accelerated on wet-ratio 33%. Corrosion resistance was superior in UHS2000 which contains alloy elements such as Ni, Cr, Mo. Formation of amorphous rust was thought to give a big effect on the corrosion resistance, that is, corrosion resistance increased with increasing the fraction of amorphous rust. The amorphous rust was thought to act as a barrier against corrosive species. The reason that corrosion was accelerated on wet-ratio 33% can be explained that the amount of amorphous rust was minimum on that condition. And the reason that UHS2000 is superior in corrosion resistance can be explained by the fact that ratio of amorphous rust was much higher in UHS2000 than in SUP7.

The effect of dominant alloy additions on the corrosion behavior of some conventional and high alloy stainless steels in seawater

The corrosion behavior of some conventional and high alloy austenitic, ferritic and duplex stainless steels has been studied at 50 °C in Gulf seawater. The alloys were tested under salt spray conditions and corrosion rates were determined by applying the electrochemical polarization resistance technique. The pitting behavior was studied using the potentiodynamic polarization technique. Both exposure and accelerated tests were employed to study crevice corrosion. The effect of surface finish on crevice attack was investigated in detail. Critical crevice solution pH (CCSpH), an important crevice corrosion parameter, was determined by a method based on the Oldfield and Sutton mathematical modelling of corrosion in chloride media [J.W. Oldfield and W.H. Sutton, Br. Corros. J. 13, 13 (1978)]. The relationship between pitting potential E pit and pitting resistance equivalent PRE N is discussed. The results of the study indicate that the presence of alloying elements such as Cr, Mo and N has a significant and beneficial influence on the pitting and crevice corrosion resistance of stainless steels.