Influence of the conversion coating on the corrosion of galvanized reinforcing steel

Galvanized reinforcing steel with a cerium conversion coating have been studied in Ca(OH)₂ saturated solution with and without chlorides. Electrochemical results reveal that cerium conversion coating provides an effective corrosion resistance compared to galvanized steel at short immersion times, 5 days without Cl⁻ ions and 2 days with Cl⁻ in the solution. The results suggest that cerium layer inhibits hydrogen evolution on the galvanized coating at early stages. At longer immersion times, galvanized steel with cerium conversion coating and galvanized steel describe similar corrosion behaviour in both electrolytes. There is not significant differences in the corrosion current density, about 5 μA/cm², due to the presence of chlorides ions in the Ca(OH)₂ saturated solution up to approximately 17 days of immersion. At longer immersion times, from 30 to 50 days, specimens in the chlorides containing solution exhibit higher corrosion activity than that recorded in the free Cl⁻ solution revealing that cerium layer cannot inhibit the localized attack promoted by chloride ions.     

Interfacial microstructures and properties of aluminum alloys/galvanized low-carbon steel under high-pressure torsion

A new composite processing technology characterized by hot-dip Zn–Al alloy process was developed to achieve a sound metallurgical bonding between Al–7 wt% Si alloy (or pure Al) castings and low-carbon steel inserts, and the variations of microstructure and property of the bonding zone were investigated under high-pressure torsion (HPT). During hot-dipping in a Zn–2.2 wt% Al alloy bath, a thick Al₅Fe₂Zn_x phase layer was formed on the steel surface and retarded the formation of Fe–Zn compound layers, resulting in the formation of a dispersed Al₃FeZn_x phase in zinc coating. During the composite casting process, complex interface reactions were observed for the Al–Fe–Si–Zn (or Al–Fe–Zn) phases formation in the interfacial bonding zone of Al–Si alloy (or Al)/galvanized steel reaction couple. In addition, the results show that the HPT process generates a number of cracks in the Al–Fe phase layers (consisting of Al₅Fe₂ and Al₃Fe phases) of the Al/aluminized steel interface. Unexpectedly, the Al/galvanized steel interface zone shows a good plastic property. Beside the Al/galvanized steel interface zone, the microhardnesses of both the interface zone and substrates increased after the HPT process.     

Prediction of fatigue resistance of a hot-dip galvanized steel

The paper clarifies the effect of a galvanizing coating on the fatigue strength of a ferritic steel. Depending on experimental conditions and on the microstructure of the coating, a reduction in fatigue strength is observed especially when the coating is thick. Cracks in the galvanizing coating rapidly form under cyclic loading and then propagate into the steel substrate. This completely modifies the distribution of crack lengths. Very short cracks are not observed in the steel when galvanized. It is shown that the propagation of a crack in the substrate from the coating is only possible when the crack completely crosses the coating. By assimilating the coating thickness to a crack in the steel substrate, the fatigue resistance of hot-dip galvanized steel can be predicted using the Kitagawa–Takahashi diagram.     

Rapid electroplating of photocatalytically highly active TiO2-Zn nanocomposite films on steel

Nanocomposite films consisting of TiO₂ and Zn with thickness of 10–15 m (TiO₂-Zn) have been electrodeposited on steel plates by rapid plating from a ZnSO₄-based bath (I _d > 10 A dm^–2). Upon addition of NH₄NO₃ to the bath (<0.3 g L^–1), the uptake of TiO₂ in the film significantly increased. Glow discharge optical emission spectrometry clarified that TiO₂ particles were incorporated throughout the film and the loaded amount increased near the surface. The first-order rate constant (k/h^–1) for gas-phase CH₃CHO oxidation was employed as an indicator of the photocatalytic activity. The k value for the TiO₂-Zn film prepared at I _d = 12 A dm^–2 (0.20 h^–1) was comparable to that for the sample from a ZnCl₂-based bath at I _d = 4 A dm^–2 (0.27 h^–1). X-ray diffraction measurements indicated that a TiO₂-ZnO nanocomposite layer was generated on the surface by the heat treatment in air at 673 K for 6 h. Consequently, the photocatalytic activity was further improved (k = 0.29 h^–1); this effect was explained in terms of the synergy of TiO₂ and ZnO in photocatalysis.     

The effects of zinc bath temperature on the coating growth behavior of reactive steel

The purpose of this work is to identify the influence of zinc bath temperature on the morphology and the thickness of reactive steel (Fe–0.1 wt.%Si alloy) coatings. The Fe–0.1 wt.%Si samples were galvanized for 3 min at temperatures in the range of 450–530 °C in steps of 10 °C. The coatings were characterized by using scanning electron microscopy/energy dispersive X-rays analysis. It was found that the coating thickness reaches the maximum at 470 °C and the minimum at 500 °C, respectively. When the reactive steel is galvanized at temperatures in the range of 450–490 °C, the coatings have a loose ζ layer on the top of a compact δ layer. With the increase of the galvanizing temperature, the ζ layer becomes looser. When the temperature is at 500 °C, the ζ phase disappears. With the increase of temperature, the coatings change to be a diffuse-Δ layer (δ+ liquid zinc).     

Dissolution kinetics of iron in liquid zinc

In hot dip galvanizing, steel strip is coated by immersion in a bath of molten zinc. The principal reactions that occur at the steel/liquid zinc interface are (1) dissolution of iron and (2) nucleation and growth of intermetallic compounds. In order to improve the management of industrial galvanizing baths, it is essential to evaluate the flux of dissolved iron that diffuses into the bath from the sheet. For this purpose, a rotating disk device has been developed to study the dissolution and diffusion of iron in pure liquid zinc at the temperature usually employed in galvanizing baths (465°C). Since the dissolution reaction is controlled by diffusion under these conditions, the diffusion coefficient of iron in liquid zinc has been measured and found to be: D _Fe ^Zn(L) = (9.8 ± 0.1) × 10^–10 m²·s^–1     

Metallurgical investigations into the genesis of bare spots,exfoliation, and matte coating appearance in hot dip galvanized steel sheets

Continuously processed hot dip galvanized steel sheets that exhibited bare spots, exfoliation/peel-off, and matte coating appearance were investigated to gain an insight into the genesis of such defects. Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDS) and electron probe microanalysis (EPMA) were employed in conjunction with analysis of the galvanizing bath. Analyses revealed a bath condition sensitive to surface cleanliness of the steel substrate and showed significant levels of carbonaceous residues and rolling debris on the annealed steel substrates. This improper steel surface condition has been attributed to excess oil carry-over on the cold rolled strip surface and poor burnoff in the annealing furnace.     

Chemical vapour deposition of corrosion-resistant TiN film to the inner walls of long steel tubes

A uniform TiN film was coated on to the inner walls of long steel tubes by moving a chemical vapour deposition furnace along the tubes. The moving direction of the furnace from outlet side to inlet side gave a more homogeneous coating than the reverse moving direction. The TiN-coated steel tube (inside diameter 10 mm) was obtained under the following conditions; peak deposition temperature 1050° C, total flow rate of reactant gas (TiCl₄ + N₂ + H₂) 6.0 ml sec^–1, moving velocity of the furnace 2.8 mm min^–1. The inner wall of the coated tube showed high corrosion resistance for dipping in 6N HCI aqueous solution for 17 h.     

Structural,electrical and optical properties of copper selenide thin films deposited by chemical bath deposition technique

A low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2–xSe thin films on glass substrates. Structural, electrical and optical properties of these films were investigated. X-ray diffraction (XRD) study of the Cu_2–xSe films annealed at 523 K suggests a cubic structure with a lattice constant of 5.697 Å. Chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). It reveals that absorbed oxygen in the film decreases remarkably on annealing above 423 K. The Cu/Se ratio was observed to be the same in as-deposited and annealed films. Both as-deposited and annealed films show very low resistivity in the range of (0.04–0.15) × 10^–5 -m. Transmittance and Reflectance were found in the range of 5–50% and 2–20% respectively. Optical absorption of the films results from free carrier absorption in the near infrared region with absorption coefficient of 10⁸ m^–1. The band gap for direct transition, E_g.dir varies in the range of 2.0–2.3 eV and that for indirect transition E_g.indir is in the range of 1.25–1.5 eV.     

Blue a.c. electroluminescence of Zn1−xMgxS:Cu,Br powder phosphors

Blue a.c. electroluminescent (EL) powder phosphors with brightness of 300 Cd/m² and half life of over 10³h can be prepared from Zn_1–x Mg _x SCu,Br. Replacement of zinc by the more electropositive magnesium increases the percentage ionicity of metal-sulphur bonds leading to the stabilization of the 2H structure. Incorporation of 15 mol% Mg shifts the EL emission peak from 525 to 436 nm. The green side band is completely absent when the copper concentration is 0.3mol%. When copper is 10^–4 mol%, both the green and blue bands are observed in the photoluminescence (PL) emission spectrum. It is proposed that hexagonal Zn_1–x Mg _x S with 0.3 mol% copper contains predominantly (Cu _Zn ^– Cu _i ⁺ ) pairs due to increased electrostatic energy. The recombination involving this isoelectronic hole centre and an isolated donor produces the blue emission. Since a large fraction of the acceptor is present as independent Cu _Zn ^– at low copper concentrations, the green band persists, which originates from distant donor (Br _S ⁺ )-acceptor (Cu _Zn ^– ) pairs.     

Optimization of the Parameters of a Porous Coating under Conditions of Pool Boiling of Propane

Results of an investigation of the influence of the parameters of a sintered porous copper coating on heat transfer under conditions of propane boiling are reported. The experiments were carried out within the ranges of specific heat fluxes of q = 10²–6.4·10⁴ W/m² and of saturation vapor pressures of p _s = 0.48–1.08 MPa (T _s = 0–30°C). Optimum parameters of the porous coating were determined within the investigated saturation pressure range. A dimensionless equation is suggested for determination of the heattransfer rate in propane boiling on sintered porous copper coatings.     

Bis-triazole as a new corrosion inhibitor for copper in sulfate solution. A model for synergistic inhibition action

Bis-[4H-5 hydroxy-1, 2, 4-triazol-3-yl] methane BHTAM was tested as a promising new inhibitor for copper corrosion in 0.5 M Na₂SO₄ solutions using open – circuit potentials and galvanostatic polarization techniques. The results show that, BHTAM is an efficient inhibitor for copper corrosion in neutral medium. The anodic polarization is strongly affected by the presence of BHTAM compound as well as the cathodic polarization but to a lesser extent. The surface condition plays a significant role in the protection of copper. The pre-formed surface cuprous oxide films facilitate inhibitor adsorption as well as precipitation of insoluble [Cu (I) – BHTAM] complex.     

Inhibition of corrosion by a mixture of nonchromate pigments in organic coatings on galvanized steel

We consider several possible substitutes for toxic chromate pigments. Among the most promising compositions for the corrosion inhibition of galvanized steel with a protective organic coating, we should mention a mixture of phosphate- and calcium-containing pigments. As was shown earlier, such a mixture possesses a high efficiency under conditions of the corrosion of zinc and galvanized steel sheet; however, it is less efficient for the protection of carbon steels. This pigment mixture forms an adsorbed phosphate film on the metal and hinders the course of anodic and cathodic reactions. Zinc ions play an important role for obtaining a synergistic protective effect, increasing the coherence and decreasing the porosity of the deposited film. Traditional and local electrochemical methods (EIS and SVE) were used for studying the corrosion of galvanized steel with coatings in acid rain solution. The data obtained corroborated the high anticorrosive efficiency of a pigment mixture in an organic coating on galvanized steel. In addition, we established some specific features of its inhibiting action near defects in the coating.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 3, pp. 74–80, May–June, 2004.     

An study on accelerated corrosion testing of weathering steel

This paper assesses the use of wet/dry cyclic laboratory corrosion tests that can provide information on the protective capacity of weathering steels in short times. Two steels were considered, a weathering steel ASTM A 242 Type 1 and a plain carbon steel (as reference), that were exposed in the atmosphere of Madrid (3 years) and in the following laboratory wet/dry cyclic tests: Cebelcor (10⁻⁴ M Na₂SO₄) (945 h), Kesternich (0.2 L SO₂) (2160 h) and Prohesion (2160 h). Characterisation of rust layers was done by XRD, FTIR, SEM, SKP and EIS. Wet/dry cyclic tests make it possible to shorten the testing time to assess in laboratory the protective capacity of rusts formed on weathering steels in the atmosphere. Some analogies between experimentation in the field and in the laboratory have been established.     

Ageing studies of discontinuous copper and silver thin films

The results of experiments carried out on the post-deposition resistance changes in discontinuous films of copper and silver with and without overlayers of SiO and Al₂O₃ are presented. The changes in the sheet resistance of the films with time and pressure were studied for the above combinations. Mobility coalescence is assumed to be responsible for the resistance increase of an uncovered copper film of initial resistance 1.9 M/. On exposure to the atmosphere, it was found that an Ag/SiO combination of initial resistance of 0.1 M/ achieved stability in the sheet resistance much quicker than a Cu/Al₂O₃ combination of initial resistance 20 M/. The fall in resistance of the Cu/Al₂O₃ composite is attributed to the formation of Al₂(OH)₆ due to the interaction of Al₂O₃ with the water vapour in atmosphere. Copper films with and without overlayers of Al₂O₃ show an abrupt increase in the sheet resistance as a function of pressure at a pressure of about 5 × 10^–2 torr with the maximum rate of change of resistance occurring at higher pressure for the higher resistance film. This indicates that the overlayer of Al₂O₃ is very porous in nature. Field effect studies were carried out on an uncovered copper film of initial resistance 10 M/ and the behaviour was found to be ohmic up to a field of 800 V cm^–1.     

Surface free energy of nickel and stainless steel at temperatures above the melting point

The surface tension (liquid-state surface free energy) of pure nickel and type 304 stainless steel was measured in a narrow temperature range above the melting point by the sessile drop method. The temperature coefficients of surface free energy in the liquid state were found to be –1.76 erg cm^–2 C^–1 for nickel and –2.48 erg cm^–2 C^–1 for stainless steel. These values are shown to be a factor of 2 larger than those previously determined for the solid surface free energies of nickel and stainless steel, but have the same sign. The latent heats of fusion of nickel and 304 stainless steel were determined by comparison of variations of solid and liquid-state surface energies with temperature and found to be 292 and 284 erg cm^–2 respectively. Measurement of the contact angle for a nickel sessile drop on thoria and a stainless steel sessile drop on alumina showed a decrease in the angle with an increase in temperature.     

Effect of chromate conversion coatings with and without poly(acrylic acid) on the press formability, shearing properties and durability of galvanized steel laminates using poly(vinyl butyral) as core material

A sandwiched type of electrogalvanized (EG) steel sheets bonded with poly(vinyl butyral) (PVB) was used to study the effect of chromate conversion coating on the interfacial bonding and bending formability of the laminate. It was found that a proper chromium passivation on the steel surface enhanced both initial tensile shearing strength (TSS) and the joint durability in the corrosive environment. The greatest TSS, i.e. 17.8 MPa, was measured for the chromated steel sheet with chromium content ranging 0.040–0.055 g/m². A good correlation between the TSS and the bending formability of the laminate was obtained. Poly(acrylic acid) (PAA) was also added in the chromate solutions and the complex conversion coating exhibited an improvement on the shearing properties of the EG steel sandwiches. The effect of PAA incorporated in the chromate coating on adhesion was dependent upon its concentration and molecular weight. The greatest TSS, i.e. 24.0 MPa, was measured by adding a proper amount of PAA with MW = 380,000 g/mol, approximately 8 wt%, in the chromate solution for EG surface coating. The possible mechanism of the enhancement of interfacial properties due to the incorporation of PAA will be discussed.     

Metadynamic recrystallization of austenitic stainless steel

Interrupted torsion tests were performed in the temperature range of 900–1100°C, strain rate range of 5.0 × 10^–2–5.0 × 10⁰/sec and interpass time range of 0.5–100 seconds to study the characteristics of metadynamic recrystallization (MDRX) for austenitic stainless steel. To compare the MDRX with static recrystallization (SRX), the pass strain was applied above the critical strain (_c) (_c = 2.2 × 10^–3 D^1/2 ₀ Z ^0.089, where Z is Zener-Hollomon parameter, Z = exp((380000 J/mol)/RT) and D ₀ is as-received grain size) to obtain the MDRX during interpass time. It was found that the kinetics of MDRX were dependent of the strain rate and deformation temperature but were nearly independent of the change in pass strain after the peak strain. The time for 50% metadynamic softening, t ₅₀, was determined as follows: t ₅₀ = 1.33 × 10^{–11 –0.41} D ₀ exp((230000 J/mol)/RT) and this calculated value was consistent with the measured value. The Zener-Hollomon parameter was impossible to evaluate the MDRX fraction, because the fractional softening values were different at the same Z values. The new parameter (MDRX parameter) considering deformation temperature, strain rate and interpass time was proposed to evaluate the MDRX fraction. The MDRX-parameter was determined as 3.25 × 10^{–19 0.3} t _i ^0.6 T ¹².     

Chemical Vapor Deposition of Copper Films from Copper Dipivaloylmethanate in Hydrogen Atmosphere

Data are presented on the kinetics and mechanism of copper deposition from Cu(dpm)₂ vapor in a hydrogen atmosphere at normal pressure. From kinetic analysis of the deposition rate as a function of substrate temperature (200–390°C) and Cu(dpm)₂ vapor pressure (7.8 Pa), the main parameters of kinetically limited deposition were determined: activation energy of 27 ± 7 kJ/mol, preexponential factor of (2.9 ± 1.8) × 10² cm/s, and reaction order of unity. Under diffusion-control conditions, the diffusion rate constant is 4.8 cm/s, and the thickness of the diffusion layer is 8 × 10^–2 cm. The films grow by a mixed (island–layer) mechanism, and their electrical resistivity is close to that of bulk copper.     

Effects of organic and inorganic treatment agents on the formation of conversion layer on hot-dip galvanized steel: An X-ray photoelectron spectroscopy study

The formation of a Cr-free conversion layer on hot-dip galvanized (HDG) steel involves various interfacial processes which radically influence the composition of the coating and thus its performance. Here, we investigate the role of the main constituents contained in the treatment bath, the polymeric compounds and manganese phosphate species, by using derivative versions from the standard solution. A detailed X-ray photoelectron spectroscopy (XPS) analysis allowed distinction between oxygen originating from polymeric compounds and oxygen of inorganic nature in oxides/hydroxides or phosphate species. Our results revealed that manganese phosphate present in the treatment bath promotes the polymer adsorption on HDG substrate. Angle-resolved XPS and depth profile showed the presence of TiO₂ layer on the outer part of the conversion layer, when polymeric compounds are not detected.