A. R. Marder

The metallurgy of zinc-coated steel

Abstract The generation of zinc and zinc alloy coatings on steel is one of the commercially most important processing techniques used to protect steel components exposed to corrosive environments. From a technological standpoint, the principles of galvanizing have remained unchanged since this coating came into use over 200 years ago. However, because of new applications in the automotive and construction industry, a considerable amount of research has recently occurred on all aspects of the galvanizing process and on new types of Zn coatings. This review will discuss the metallurgy of zinc-coated steel from a scientific standpoint to develop relationships to practical applications. Hot-dip zinc coating methods, i.e. batch and continuous processes, will first be reviewed along with Fe–Zn phase equilibria and kinetics. Commercially, the addition of aluminum to the zinc bath results in three important types of coatings, galvanized, galfan and galvalume, and produces complex reactions at the coating/substrate interface. Fe–Zn–Al equilibrium will be reviewed in the light of recent studies of solubility and inhibition layer formation and breakdown. The effect of steel substrate composition on these reactions will also be critically analyzed. The overlay coating formation, or the coating alloy, is specifically chosen for its desired properties. The morphology of the galvanize, galfan and galvalume coating overlays will be reviewed, as well as the effect of heat treatment to produce a galvanneal coating. Finally, the effect of the microstructures of these coatings on the important properties of corrosion, formability, weldability and paintability will be discussed.

Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450°C: Part II 0.20 wt% Al-Zn baths

The effect of solute additions of titanium, titanium and niobium and phosphorus on interstitial-free steels on Fe-Zn phase formation after immersion in a 0.20 wt% Al-Zn bath was studied to determine the morphology and kinetics of the individual Fe-Zn phases formed. These results were contrasted to the previous study using a pure zinc (0.00 wt% Al) bath in Part I. It was found that in the 0.20 wt% Al-Zn bath, an iron-aluminide inhibition layer prevented uniform attack of the steel substrate. Instead, localized Fe-Zn phase growth occurred, termed outbursts, containing a two-phase layer morphology. Delta-phase formed first, followed by gamma-phase. Zeta-phase did not form in the 0.20 wt% Al-Zn bath, in contrast with zeta-phase formation in the pure zinc bath. As in the pure zinc bath, the growth kinetics of the total layer was controlled by the Fe-Zn phase in contact with the liquid zinc during galvanizing. For the 0.20 wt% Al-Zn bath, the Fe-Zn phase in contrast with the liquid zinc was the delta-phase, whereas the zeta-phase was in contact with liquid zinc in the pure zinc bath. The delta-phase followed t1/2 parabolic growth, while the gamma-phase showed essentially no growth after its initial formation. Titanium and titanium + niobium solute additions, which enhance grain-boundary reactivity, resulted in more rapid growth kinetics of the gamma- and delta-phases. Phosphorus additions, which decrease grain-boundary reactivity, generally increased the incubation time and retarded the growth rate of the gamma-phase. These results further confirm the concept that solute grain-boundary reactivity is primarily responsible for Fe-Zn phase growth during galvanizing in a liquid Zn-Al bath in which an iron aluminide inhibition layer forms prior to Fe-Zn phase formation.

The effect of velocity on the solid particle erosion rate of alloys

Abstract Velocity is a critical test variable in erosion, and can easily overshadow changes in other variables, such as target material, impact angle, etc. The effect of velocity on erosion rate was studied in 70–30 brass (cold worked and annealed) and Fe–C martensite (as-quenched and tempered) and it was found, as previously shown, that erosion rate is dependent on velocity by a power law, given by ER= kV n . However, the velocity exponent n was found to be target material independent and is governed by test conditions, including particle characteristics and the erosion test apparatus. In addition, n is not dependent on the erosion mechanism. Results from tested as-quenched martensite and tempered martensite showed that the exponent is approximately 2.9 for both materials, even though martensite eroded by a brittle cracking mechanism, while tempered martensite eroded by a plastic deformation mechanism. No difference in the erosion rate relationship was found between the fully annealed and the 70% cold worked brass. The exponent n was found to change over time with nominally the same erosion test conditions, indicating that n is very sensitive to slight changes in erodent particles and/or the test apparatus, and that it must be measured periodically if erosion results generated at different times are to be compared. 1

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. II: Oxidation Kinetics and Morphology at 1000°C

The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of γ (Ni)+γ ′(Ni3Al). During short-term exposure at 1000°C, a thin α-Al2O3 layer developed below a matrix of spinel NiAl2O4, with θ-Al2O3 needles at the outer oxide surface. After 100 hr of oxidation, remnants of θ-Al2O3 are present with spinel at the surface and an inner layer of α-Al2O3. After 1000–2000 hr, a relatively thick layer of α-Al2O3 is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al2O3 layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al γ -alloys exhibit a healing Al2O3 layer after transient Ni-rich oxide growth. Higher Al alloys display Al2O3-controlled kinetics with low mass gain during TG analysis.

Modeling solute redistribution and microstructural development in fusion welds of Nb-bearing superalloys

Abstract Solute redistribution and microstructural evolution have been modeled for gas tungsten arc fusion welds in experimental Ni base and Fe base Nb-bearing superalloys. The multi-component alloys were modeled as a ternary system by grouping the matrix (Fe, Ni, Cr) elements together as the “solvent” to form the γ component of the γ-Nb–C “ternary system”. The variation in fraction liquid and liquid composition during the primary L→γ and eutectic type L→(γ+NbC) stages of solidification were calculated for conditions of negligible Nb diffusion and infinitely fast C diffusion in the solid phase. The proposed model is based on modifications of solute redistribution equations originally developed by Mehrabian and Flemings. Results of the calculations were superimposed on the pseudo-ternary γ-Nb–C solidification surfaces to predict the solidification reaction sequences along with the total and individual amounts of γ/NbC and γ/Laves eutectic-type constituents which form during solidification. Comparison was made to experimentally measured values, and reasonable agreement was found. The model results permit quantitative interpretations of composition–microstructure relations in these Nb-bearing experimental alloys and should provide useful insight into comparable commercial alloy systems as well.

Electrodeposited NiAl particle composite coatings

Abstract Metal matrix/metal particle composite coatings were deposited, heat treated and characterized. A nickel matrix/aluminum particle coating deposited from a sulfamate bath was chosen as a model system. By varying the process parameters, up to approximately 20 vol. % Al particles were incorporated into the coatings. Based on light optical and scanning electron microscopy, a schematic model is presented for coating morphology development during codeposition of small, electrically conductive particles. For high deposition current densities, the hardness of the as-plated Ni-Al coating was higher than that of pure Ni coatings due to the nucleation and growth of small Ni grains on the surface of the Al particles. This structural refinement leads to Vickers hardness of about 350–400 HVN (25 g load) for Ni Al coatings compared to about 200–250 HVN (25 g load) for pure Ni coatings deposited under similar conditions.

Erosion of thermal spray MCr–Cr3C2 cermet coatings

High velocity oxy fuel (HVOF) cermet coatings have been used to reduce the damage caused by solid particle erosion (SPE). The current work investigated the erosion resistance of FeCrAlY–Cr3C2 and NiCr–Cr3C2 cermet coatings with carbide levels ranging from 0–100% (in the pre-sprayed powder) in order to determine the optimum ceramic content for the best erosion resistance. The as sprayed coating microstructures were analyzed using light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction, and microprobe techniques. Erosion testing was carried out using a particle accelerator apparatus set at a velocity of 40 m/s, a mass flow of 80 g/min, and 400°C with 450 μm Al2O3 particles at 30° and 90° impact angles. Image analysis revealed the carbide levels in the pre-sprayed powder are much higher than in the final as sprayed coating. The lower carbide levels in the coatings are caused by a combination of poor carbide spray efficiency and reduction/oxidation of the carbide particles in the HVOF jet, resulting in the formation of various oxides and metal rich carbides. Erosion testing of the sprayed coatings found that decreasing the carbide content, and overall hard phase content (oxides and carbides), decreased the erosion rate for 90° impact. In addition, for 30° impact the erosion rate remained fairly constant regardless of carbide or hard phase content.

Characterization of single and discretely-stepped electro-composite coatings of nickel-alumina

Electrodeposition from a sulfamate bath has been used to produce single layer and discretely stepped electro-composites consisting of a metallic nickel matrix with second phase alumina (α-Al2O3) particles. Light optical microscopy (LOM), scanning electron microscopy (SEM), quantitative image analysis (QIA), and micro-indentation techniques were used to characterize the deposits. As previously seen, an increase in bath particle loading and decrease in plating current density increased the volume percent of alumina incorporated into the coating, with a maximum of 40 vol % being attained. For samples deposited above 1 A/dm2, a direct relationship between the alumina volume percent and coating hardness was seen due in part to the related decrease in interparticle spacing (IPS) at the higher vol %. However, the strengthening mechanism of the electro-composites may be more complex with both the metallic nickel grain structure and IPS being factors, as seen for samples deposited at 0.5 A/dm2. The incorporation of alumina into the electrodeposited nickel was also observed to affect the as-plated surface structure of the coating. Due to the particles inhibiting the formation of pyramidal features found on the surface of pure nickel electrodeposits, the electro-composite surfaces were observed to be relatively flat. Also, structure within the metallic nickel matrix appeared due to rapid growth of the nickel coating around the inert particles when plated at high current densities. In addition, discretely layered functionally graded materials were produced without alterations to the original deposition procedure of the single layer deposits. It was found that the various processing stages needed to produce the stepped coatings did not affect the structure or properties of the individual layers, when compared to that of the corresponding single-layered electro-composites.

Microstructural characterization and hardness of electrodeposited nickel coatings from a sulphamate bath

Pure nickel plates were produced from a sulphamate bath by electrodeposition. A systematic variation of the current density, in the range 0.005 to 0.25 Acm−2, resulted in a variation of the coating microstructure and properties. Deposits plated below a current density of 0.01 Acm−2 had a surface morphology consisting of large, deep crevices surrounding smaller substructures. A banded or laminar type microstructure was observed in cross-section. Above this current density, truncated pyramidal structures, with ridged terraces oriented perpendicular to the growth direction, were found on the surface. The planar dimensions of the pyramidal surface features were found to increase with current density, as well as the columnar grain widths observed in the cross-sectional view. To evaluate the mechanical properties of the coatings, microindentation hardness tests were performed using a Knoop indenter. A Hall–Petch type relationship for the samples deposited at and above 0.01 Acm−2 was seen.

Ni–Al composite coatings: diffusion analysis and coating lifetime estimation

Abstract The interdiffusion of Ni matrix/Al particle composite coatings and nickel substrates was studied using electron probe microanalysis (EPMA) and a one-dimensional diffusion model. The initial coating microstructure was a two-phase mixture of γ(Ni) and γ′(Ni 3 Al). The coating/substrate assemblies were aged at 800 to 1100°C for times up to 2000 h. It was found that aluminum losses to the substrate are significant at 1000°C and above. The experimental results for the diffusion of Al into the substrate were compared with model predictions based on a diffusion equation for a finite layer on an infinite substrate. Using combined experimental and model results, the effects of temperature and coating thickness were determined and a rationale was developed for coating lifetime prediction.