S. B. Lyon

The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and growth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputtering (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerated 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The coating and defects were then examined in planar view and cross-section by scanning electron microscopic analyses and the results were compared with those prior to the electrochemical measurement. It is found that the overall coating/substrate corrosion process is closely related to the deleterious effect of the macroparticles and growth defects in the PVD coatings. It is further demonstrated that for through thickness macroparticle inclusions, corrosion initiates by galvanic or crevice corrosion between the defect and the coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate interface. On the basis of the experimental findings and the macroparticle formation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed

Combined cathodic arc/unbalanced magnetron grown CrN/NbN superlattice coatings for corrosion resistant applications

Abstract CrN/NbN superlattice coatings have been developed as a potential alternative to electroplated hard chrome for specialised industrial applications. The coatings have been grown at 400°C by combined cathodic arc/unbalanced magnetron technique. Two types of coating, stoichiometric (N/Me=1) and substoichiometric (N/Me=0.5) with superlattice wavelength 3.7xa0nm and 6.5xa0nm, respectively have been deposited on single-phase CrN and two-phase Cr 2 N+CrN; 0.35xa0μm thick base layer. It has been found that the residual stress levels in the CrN/NbN superlattice coating depend on the chemical composition as well as the phase composition of a monolithically grown base layer. The corrosion resistance of CrN/NbN superlattice coatings has been investigated by potentiodynamic polarisation measurements in aerated acetic acid/sodium acetate buffer solution and compared to 20xa0μm thick electroplated chromium as well as 304L stainless steel. CrN/NbN coatings showed clear passivation behaviour with pitting potentials in the range 230xa0mV to 400xa0mV depending on their chemical composition, residual stress levels and CrN base layer phase composition. The lower stressed stoichiometric CrN/NbN superlattice coating deposited on a single-phase CrN base layer showed the best performance with passive current density as low as 0.35xa0μAxa0cm −2 . In contrast the electroplated chromium did not show any evidence of passivity with a factor of 10 higher corrosion current density when compared to stoichiometric CrN/NbN superlattice coating.

The influence of low concentrations of chromium and yttrium on the oxidation behaviour, residual stress and corrosion performance of TiAlN hard coatings on steel substrates

Ti0.43Al0.52Cr0.03Y0.02N films, which have been shown to exhibit a fine grain near equiaxed microstructure were found to exhibit a compressive residual stress of - 6.5 GPa in contrast to conventional columnar Ti0.44Al0.53Cr0.03N coatings which demonstrated - 3.8 GPa compressive stress. Novel coatings with this modified microstructure were also found to possess improved resistance to both dry oxidation and wet aqueous corrosion. Glancing angle parallel beam geometry X-ray diffraction (GAXRD) studies showed that in conventional Ti0.44Al0.53Cr0.03N films, severe oxidation initiated above 850 degrees C whilst oxidation of Ti0.43Al0.52Cr0.03Y0.02N started close to 950 degrees C. In an alkaline aqueous medium, Ti0.43Al0.52Cr0.03Y0.02N coatings deposited on steel showed an extended passive potential range and a significantly lower passive current compared with Ti0.44Al0.53Cr0.03N films of similar thickness. A similar improvement was evident in sulphuric acid where yttrium containing coatings passivated at high potential (Ti0.44Al0.53Cr0.03N films did not passivate). These effects may be ascribed to reduced porosity in the fine-grained Ti0.43Al0.52Cr0.03Y0.02N as well as the well-known effects of low concentrations of yttrium on high-temperature oxidation performance

Corrosion performance of CrN/NbN superlattice coatings deposited by the combined cathodic arc unbalanced magnetron technique

Abstract Potentiodynamic polarisation experiments have been used to evaluate the corrosion performance of novel CrN/NbN superlattice coatings in comparison with a commercial CrN coating and a 304L control specimen. In addition, electrochemical impedance has been used to determine the effective surface area of the coatings by capacitance measurements. Results indicate that the superlattice coatings have significantly improved barrier properties evidenced by increases in pitting potentials of up to 500 mV compared with 304L. Furthermore, the impedance evidence indicates that CrN has comparatively large pores while the superlattice coatings have extensive, but diffuse and fine-scale, surface porosity confirming that the corrosion performance improvement is most likely due to microstructural enhancements.

The effect of strontium and chromate ions on the inhibition of zinc

Abstract The corrosion and inhibition of zinc in artificial acid rain solution (pH 4.5) at a low strontium chromate concentration has been evaluated using electrochemistry, solution analysis and XPS. These techniques have been used to estimate inhibitor efficiency and to determine a possible inhibition mechanism. The results show that strontium chromate at low concentration inhibited significantly more of the cathodic corrosion process, by reduction of Cr6+ to a solid species containing Cr3+. The surface film contained a mixture of oxidised zinc and chromium, probably in the form of basic zinc carbonate (hydrozincite) and hydrated chromia. Importantly strontium ions were found to play no significant role in the inhibition process.

Wear associated with growth defects in combined cathodic arc/unbalanced magnetron sputtered CrN/NbN superlattice coatings during erosion in alkaline slurry

The erosive wear associated with growth defects was studied in deaerated 0.5 M (Na2CO3-NaHCO3) buffer solutions containing 150-200 mum alumina particles on a rotating cylinder erosion-corrosion system for PVD CrN/NbN superlattice coatings grown by the are bond sputtering (ABS) process on mild steel BS6323. Corrosion was minimised by selective control of the samples potential, according to potentiodynamic polarisation, during the slurry erosive wear test. The morphology of the coatings, particularly of the defects, was examined in planar and cross-section views by means of scanning electron microscopy before and after the test. It is found that wear of the coating is typically preferential to the defects, progressing from the particle exterior top to the interior with erosion time, while the coating matrix (areas free of such defects) is largely intact, after 24 h prolonged exposure to erosion

Electrochemical impedance spectroscopy of PVD-TiN coatings on mild steel and AISI316 substrates

The electrochemical and corrosion behavior of PVD-TiN coatings was investigated through electrochemical impedance spectroscopy (EIS) in a 3% NaCl solution. Mild steel (active) and AISI316 stainless steel (passive) substrates were employed. The results indicated that it was possible to follow the corrosion of the active substrate using EIS over a period of 500 h immersion. The AISI316 substrate, however, remained effectively passive for the duration of the experiment. Based on this study, the EIS technique certainly shows promise in assessing the corrosion performance (and the coating quality) of electron-beam plasma-assisted PVD-TiN coatings on active substrates such as mild steel. Its usefulness on passive materials appears more limited, owing to the similarity of the electrochemical impedance response of the coating and substrate.

An a.c. impedance study on PVD CrN-coated mild steel with different surface roughnesses

CrN coatings produced by physical vapour deposition (PVD) technology on mild steel of different surface roughness have been characterized using a.c. electrochemical impedance spectroscopy (EIS). It was found that as the roughness of the substrate decreased, the number of flaws, evidenced by the frequency and size of initial corrosion sites, also decreased. Thus it was concluded that coating porosity controlled the initiation of corrosion on the underlying substrate. In addition, the coating adhesion increased substantially as the substrate roughness decreased. As corrosion was seen to progress via an undermining mechanism with volume expansion and subsequent cracking of the coating, it was concluded that coating adhesion greatly influenced the propagation of corrosion on PVD-coated metals.

The Partial Ionic and Electronic Conductivity of Y-Containing and Y-Free Chromia Scales

This work reports on the ionic and electronic partial conductivity, determined in situ by the Wagner asymmetry-polarization technique, of chromia scales thermally grown in 1 atm flowing oxygen at 900°C. Results show an activation energy of 0.3 eV for ionic conductivity and of 0.6 eV for electronic conductivity in the temperature range 300–700°C. After treatment with yttria, both the ionic and electronic conductivity were reduced; however, the activation energy was the same as the Y-free scales. This is consistent with previous results obtained from chromia scales grown in H2/H2O vapor. Comparison of the data between scales grown at low and high PO2 suggests that hydrogen doping in the scale can reduce both ionic and electronic conductivity by approximately one order of magnitude.

Preferential erosive wear of droplet particles for cathodic arc/unbalanced magnetron sputtering CrN.NbN superlattice PVD coatings

In a previous letter [1], we have demonstrated that corrosion of a physical vapor deposition (PVD) coating substrate system can be induced by coating defects, resulting in premature damage to the usually hard and corrosion resistant coatings. Shrinkage pin holes, one of the most common defects, allow access of solution to substrate to cause galvanic corrosion [2, 3], and this may be prevented by forming a pin-hole interrupting under or sandwich layers of passivating metals such as Al or Ti [4, 5]. Since the increasing application and development of plasma arc in various PVD techniques, owing to the capability of the arc technique to achieve relatively high metallic vapor ion valence states, surface and atom mobility and diffusivity, and consequently the greater coating-substrate adhesion [6], more attention has been paid to the study of the arc-related formation of droplet or macroparticle coating defects.Due to the nature of its formation, a macroparticle is the product of a droplet of cathode metal induced by plasma arc heating, which, after being ejected from the cathode, solidifies and becomes embedded within the coating after incomplete reaction (due to its size) within the deposition chamber gas (N-plasma, for example)during its flight to the substrate [7]. That is why a macroparticle is different, in addition to its great size,in chemical composition (N-content) not only from the adjacent coating matrix, but also from the particle exterior to interior parts [8]. The composition inconsistency of the droplets rendered the defects to galvanic coupling (anodic to the adjacent coating matrix) to corrode first upon contact with aqueous solutions, and with progression, this eventually led to the penetration of solutions to the substrate to cause more severe crevice corrosion [9]. The severity of such droplet-induced corrosion depends on many factors but generally droplets are regarded to be detrimental as far as their effect on the overall coatings corrosion performance is concerned [10-12].