Slurry erosion–corrosion behaviour of high-velocity oxy-fuel (HVOF) sprayed Fe-based amorphous metallic coatings for marine pump in sand-containing NaCl solutions

Corrosion and slurry erosion–corrosion (E–C) behaviour of FeCrMoMnWBCSi amorphous metallic coatings (AMCs) compared with 304 stainless steel was investigated by static electrochemical measurements and weight loss tests under rotating conditions in simulated seawater. E–C rates of the AMCs increased with flow velocity, particle size, sand content and NaCl concentration. The AMCs are preferentially attacked at coating defects. The superior E–C resistance of AMCs can be attributed to the high micro-hardness, alloying elements and amorphous microstructure. The AMCs with better slurry E–C resistance may be good candidates to solve the E–C problems of marine pumps in sand-containing seawater.     

Oscillational corrosion potential of HastelloyC coatings fabricated by GS-HVOF spraying

Coatings of HastelloyC fabricated by HVOF spraying with a gas shroud (GS) have shown the superior barrier characteristic and corrosion resistance in seawater environment. During immersion of these coatings in artificial seawater, however, oscillation phenomenon of the corrosion potential was observed. In order to reveal and control the oscillation behaviour, some types of surface modification of the sprayed coatings and changing of the spray condition were carried out and their effect on the corrosion potential was investigated. The oscillation was caused by repetition of seawater penetration through surface oxides of the sprayed particles to their metal face and subsequent passivation of the metal. Such surface oxides were formed upon the stacking process of the sprayed particles on the substrate. Surface modification of the sprayed coatings and changes of the spray condition could reduce the oscillation effectively.     

Effect of porosity sealing treatments on the corrosion resistance of high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous metallic coatings

High-velocity oxy-fuel-sprayed FeCrMoMnWBCSi amorphous metallic coatings were sealed with sodium orthosilicate (Na₃SiO₄), aluminium phosphate (AlPO₄), and cerium salt sealants. The microstructure of the sealed coatings was characterised by scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction. Corrosion behaviour was examined using electrochemical methods of potentiodynamic polarisation, cyclic polarisation, electrochemical impedance spectroscopy, and Mott-Schottky tests. The results indicated that the uniform corrosion resistance of the three sealed coatings was enhanced greatly, and the passive current densities were decreased by one order of magnitude after the sealing treatments. The AlPO₄ sealant can penetrate the coatings by no less than 50 μm and enhance their hardness, which exhibited a more uniform corrosion resistance, fairly good pitting corrosion resistance, and can be applied in long-term corrosive and/or abrasive environments. The cerium salt-sealed coating showed better pitting corrosion resistance but inferior corrosion resistance in the local regions of micro-cracks, which was practically used for temporary corrosion protection. The Na₃SiO₄-sealed coating showed better uniform corrosion resistance and inferior pitting corrosion resistance, which can be applied in short-term corrosion environments. The stability of the passive film affected the corrosion behaviour of the sealed coatings. The AlPO₄-sealed coating performed better as a protective passive film during the long-term immersion test for a lower defect concentration and a more protective passive film.     

Improving the corrosion and tribocorrosion resistance of Ni–Co nanocrystalline coatings in NaOH solution

Ni–Co nanocystalline coatings were electrodeposited from a modified Watts bath. Increasing the deposition current density had no significant effect on structure, corrosion and tribocorrosion behavior of the coatings. Adding saccharin into the bath reduced the grain size, increased the hardness, changed the texture component from (2 0 0) to (1 1 1), smoothed the surface morphology, increased the corrosion resistance and improved the tribocorrosion behavior of coating. Presence of sodium lauryl sulfate in the bath increased the corrosion resistance of coating by producing a more compact surface morphology. However, the coating showed low tribocorrosion resistance, probably due to its lower hardness.     

Enhanced corrosion protection of MgO coatings on magnesium alloy deposited by an anodic electrodeposition process

MgO coatings were prepared on magnesium alloy surface by an anodic electrodeposition process in concentrated KOH solution followed by heat treatment in air. The phase composition and microstructure of the as-formed MgO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the MgO-coated samples was evaluated by electrochemical measurements and immersion tests in Hanks’ solution. The results showed that the MgO-coated Mg alloy exhibited a much superior stability and lower corrosion rate, and thus enabled to improve the corrosion resistance, whereas the bare Mg alloy suffered from severely localized corrosion attack.     

Microbial corrosion resistance of galvanized coatings with 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one as a biocidal ingredient in electrolytes

Electrodeposition of galvanized coatings from electrolyte containing 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) can increase microbial corrosion resistance. Coatings were found to inhibit the growth and metabolism of sulphate-reducing bacteria (SRB). Open circuit potentials and corrosion rates of coupons revealed DCOIT effectively influences the coating property. Energy diffraction spectrum and infrared absorption spectra were used to detect DCOIT on the coating surface. Scanning electron microscopy and X-ray diffraction revealed morphological and structural modifications. Electrochemical impedance spectroscopy and polarization techniques determined the corrosion behaviour of coatings in SRB. Results showed coatings formed from electrolytes with DCOIT have improved microbial corrosion resistance and bactericidal action.     

Effects of grain size on the electrochemical corrosion behaviour of electrodeposited nanocrystalline Fe coatings in alkaline solution

Effect of grain size reduction on the electrochemical corrosion behaviour of nanocrystalline Fe was investigated using Tafel polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Nanocrystalline iron was fabricated by pulse electrodeposition using citric acid bath. The grain size of a nanocrystalline surface was analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The corrosion resistance of Fe in alkaline solution considerably increased as the grain size decreased from microcrystalline to nanocrystalline. The behaviour of passive film growth and corrosion was discussed in terms of excess of free energy caused by nanocrystalline surface.     

Microstructure and corrosion resistance of modified 2024 Al alloy using surface mechanical attrition treatment combined with microarc oxidation process

A top ceramic coating was fabricated on the surface mechanical attrition treatment (SMAT) modified nanocrystalline layer of 2024 Al alloy by microarc oxidation (MAO) process. The corrosion resistance of the SMAT-MAO composite coating was studied by EIS. The results show that SMAT-MAO composite coating with 10 μm top ceramic coating exhibited better corrosion resistance, while the SMAT-MAO coating with the thickness of 15 μm showed worse corrosion resistance compared with those simple MAO coatings with same thickness. The formation of a dense passive film at the damaged region caused by the bottom nanocrystalline interface contributed to the improved corrosion resistance property.     

Corrosion resistance of amorphous aluminium-molybdenum alloys in an acidic chloride environment

Pure aluminium corrodes readily in 1 M HCl solution while its supersaturated alloys with Mo, explored in this work, exhibit excellent corrosion properties due to the oxide barrier film formation on the alloy surface. The influence of Mo content in the alloy on the potential and charge distribution across the system alloy |oxide film| solution was studied in situ using dc and ac electrochemical methods. Passivity, a sudden change in electrode impedance, and the onset of secondary passivity and pitting were discussed in terms of the solute segregation, according to the solute-rich mechanism, change in the alumina structure and solid state processes connected with the transport of point defects and spatial distribution of surface charge.     

Influence of salt spray corrosion on subsequent isothermal oxidation behaviours of AIP NiCoCrAlYSiB coatings

The influence of salt spray corrosion on subsequent oxidation behaviours of arc ion plated (AIP) NiCoCrAlYSiB coatings is investigated in this work. The results indicate impact of the salt spray corrosion on successive oxidation behaviour depends greatly on the degree of the previous corrosion attack. Accelerated oxidation kinetics and unsatisfactory oxide morphologies are observed on the as-deposited and as-annealed NiCoCrAlYSiB coatings that suffered serious salt spray attacks. Pre-oxidised coatings prove excellent performances during both salt spray tests and the subsequent oxidations. The mechanism of the corrosion attack influencing the isothermal oxidation behaviour is discussed.     

EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings

The possibilities offered by the EIS technique for studying the progress of corrosion in several types of galvanised coatings were investigated. Corrosion tests were performed in a 3 wt.% NaCl aqueous solution, at approximately neutral pH, without stirring and in contact with the air. A tendency was seen towards an electrochemical overestimation of the corrosion values of essentially pure zinc and Zn-5%Al (Galfan) coatings compared with the gravimetric reference values. In contrast, the opposite tendence was revealed with the Zn-10%Fe (Galvanneal) coating. These deviations have made it necessary to use empirical values of the constant B of the Stern-Geary equation. The approximately linear progress of the attack with immersion time suggests a slight barrier effect of the corrosion products layer.     

Corrosion resistance of HVOF sprayed HastelloyC nickel base alloy in seawater

The nickel base alloy, HastelloyC, deposited by HVOF thermal spraying, was investigated to ascertain the characteristics as a protective coating for the structural steels under the marine environment. Its corrosion resistance was evaluated in addition to its physical and its chemical properties. The HastelloyC deposit with higher impermeability had a comparatively higher corrosion resistance in artificial seawater. It possessed a nature of general corrosion because of its contamination by oxidation of flying particles during the spray process. In the case of the porous deposit, a reaction similar to the crevice corrosion took place on pores between its sprayed particles.     

Some aspects in designing passive alloys with an enhanced corrosion resistance

The technical and biomedical use of alloys containing reactive metals in aggressive environments is possible due to the spontaneous formation of thin oxide layers which slow down corrosion reactions by several orders of magnitude. Oxide films of important technical metals like Cr and Cu are cation semiconductors, and therefore they are electrochemically unstable and susceptible to anodic decomposition in the contact with electrolyte solutions. Alloying with elements that easily undergo oxidation and formation of highly charged cations, which interact electrostatically with the mobile cation vacancies, results in an enhanced electronic conductivity of their oxide films.This is an effective way how to design an alloy with significantly greater corrosion resistance in aggressive environments in comparison with its constitutive elements. The present paper describes several selected systems investigated under conditions of their real application.     

Effect of molybdate post-sealing on the corrosion resistance of zinc phosphate coatings on hot-dip galvanized steel

The technique of post-sealing the phosphated hot-dip galvanized (HDG) steel with molybdate solution was addressed. The composition and corrosion resistance of the improved phosphate coatings were investigated by SEM, EDS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements, and neutral salt spray (NSS) test. The results showed that molybdate films were formed in the pores of phosphate coatings, and the compact and complete composite coatings composed of phosphate coatings and molybdate films were formed on the zinc surface, resulting in that both the anodic and cathodic processes of zinc corrosion were inhibited remarkably; the corrosion protection efficiency values were increased; and the electrochemical impedance values were enhanced at least one order of magnitude. The low frequency impedance values for the composite coatings were increased at the initial stages of immersion in 5% sodium chloride solution, indicating the self-repairing activity of the composite coatings.     

Corrosion resistance of zinc-magnesium coated steel

A significant body of work exists in the literature concerning the corrosion behaviour of zinc-magnesium coated steel (ZMG), describing its enhanced corrosion resistance when compared to conventional zinc-coated steel. This paper begins with a review of the literature and identifies key themes in the reported mechanisms for the attractive properties of this material. This is followed by an experimental programme where ZMG was subjected to an automotive laboratory corrosion test using acidified NaCl solution. A 3-fold increase in time to red rust compared to conventional zinc coatings was measured. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the corrosion products formed. The corrosion products detected on ZMG included simonkolleite (Zn₅Cl₂(OH)₈ · H₂O), possibly modified by magnesium uptake, magnesium hydroxide (Mg(OH)₂) and a hydroxy carbonate species. It is proposed that the oxygen reduction activity at the (zinc) cathodes is reduced by precipitation of alkali-resistant Mg(OH)₂, which is gradually converted to more soluble hydroxy carbonates by uptake of atmospheric carbon dioxide. This lowers the surface pH sufficiently to allow thermodynamically for general precipitation of insoluble simonkolleite over the corroding surface thereby retarding the overall corrosion reactions, leaving only small traces of magnesium corrosion products behind. Such a mechanism is consistent with the experimental findings reported in the literature.     

Corrosion of zinc–magnesium coatings: Mechanism of paint delamination

Due to their promising corrosion properties, metallic coatings containing magnesium are currently widely investigated for use as protective coatings for steel sheet. Particularly, alloying zinc coatings with magnesium results in a remarkable improvement of the corrosion resistance of the painted system. While some aspects of this improvement have been understood, the progress of the corrosive degradation of the alloy coating/paint interface has not been reported in detail. In this paper, the delamination of a model polymer from the intermetallic MgZn₂ is described and a degradation mechanism proposed. Aspects for the design of stable interfaces are discussed.     

Corrosion protection of aluminum pigments by sol-gel coatings

New sol-gel coatings for the corrosion protection of aluminum pigments were developed. To this end, the pigment particles are first coated with a silica layer by phosphoric acid-catalyzed sol-gel processing of Si(OEt)₄, to which either hexadecyltrimethoxysilane or dimethyldimethoxysilane is condensed. The coated pigments have excellent anticorrosive properties in alkaline solutions as well as under boiling water conditions.     

Direct-to-metal UV-cured hybrid coating for the corrosion protection of aircraft aluminium alloy

Finding eco-efficient and environmentally viable alternatives to chromate coatings represents a fundamental milestone in the aerospace industry. Here, we show a chromate-free approach to protective hybrid coatings on aluminium alloy (AA2024-T3) departing from photoinduced sol–gel and cationic polymerizations. Beginning with a film of n-alkyltrimethoxysilane and diepoxy monomer, we rely on photogenerated superacids to induce the single step formation of two inorganic and organic barrier networks. Such system combines the unique aspects of photopolymerization including fast reactions, temporal control, solvent-free composition and temperature independence. Used without chemical conversion coating or anodizing, some films have passed 2000 h of salt spray testing.     

Direct electrosynthesis of polyaniline–montmorrilonite nanocomposite coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline–montmorrilonite (MMT) nanocomposite coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) by using the galvanostatic polarization method. The synthesized coatings were characterized by UV–Vis absorption spectrometry, Fourier transform infrared spectroscopy, X-ray diffraction patterns and scanning electron microscopy. The corrosion protection effect of the coatings was demonstrated by performing a series of electrochemical experiments of potentiodynamic and impedance measurements on Al in 3.5 wt% aqueous NaCl electrolytes. The corrosion current (i_corr) values decreased from 6.55 μA cm⁻² for uncoated Al to 0.102 μA cm⁻² for nanocomposite-coated Al under optimal conditions.     

Photocurrent and capacitance investigations into the nature of the passive films on austenitic stainless steels

Photocurrent and capacitance measurements of semiconductor passive films formed on metals and alloys can be used to study the electronic properties and reveal indirect information about structure and composition. The current work used these techniques to investigate the electronic properties of the passive films formed on three austenitic stainless steels, types 304L, 316L and 254SMO, in borate. Evidence was found for the existence of a large number of localised mid bandgap states, consistent with amorphous oxides. However, the flat-band potentials of the austenitic stainless steel passive films were found to be independent of both composition and measuring frequency. The most credible explanation for the bandgap values determined from photocurrent measurements is that the passive films are formed as dual layers, iron oxide outer layer and chromium oxide inner layer. This model does not need to evoke the potential dependent bandgaps used by previous authors.