Corrosion Resistance Of Zn Research Articles

Effect of rare earth metals on microstructure and corrosion resistance of Zn–0·18Al coatings

Effect of rare earth (RE) metals addition on the microstructure, formation of Fe–Zn intermetallics and corrosion resistance of the batch galvanising Zn–0·18Al coating were studied. Microstructure of the coating was observed using optical microscopy, scanning electron microscopy and transmission electron microscopy. Salt spray test and surface potential measurement were employed for corrosion resistance determination. The results show that the addition of RE can improve bath liquidity, refine surface spangles, stabilise the Fe2Al5 inhibition layer, decrease coating thickness and enhance corrosion resistance of the coating. However, corrosion resistance of the coating did not increase continuously with increasing RE content and the optimum RE content lies between 0·045 and 0·069% in the experiment. Mechanism of RE on the corrosion resistance of the galvanised coating was briefly analysed.

Electroplating of Zn Coating on AZ31 Magnesium Alloy in ZnF2 Solution

Zn electrodeposited directly on the surface of AZ31 Mg alloy was researched in this paper. Zinc fluoride which was dissolved in ammonium citrate solution was used as metal ion source. The interface and morphology of Zn coating were observed by scanning electron microscopy (SEM).The results showed that Zn coating was smooth and dense. The experiment of making lattice on Zn coating showed good adhesion. The corrosion resistance of Zn coating was carried out in 5% NaCl solution, and its stabilization time was more than 5hrs. Cyclic voltammetry and polarization curve were used to investigate the transfer behavior and state of zinc ion in electrolyte. It indicated that Zn existed in the forms of three complex states in electroplating solution and ZnL proportion increased with increasing pH value. It was known that complex Zn electrodeposited directly on the cathode.

Understanding corrosion via corrosion product characterization: I. Case study of the role of Mg alloying in Zn–Mg coating on steel

In the present work the corrosion inhibitive role of Mg in Zn–Mg coatings is considered for different stages of corrosion. Corrosion product characterization was carried out using XRD, IRRAS, MEB–FEG–EDS on technical Zn–Mg coatings after various exposure times in a standardized cyclic corrosion test. The results are compared with artificial corrosion products obtained by chemical and electrochemical synthesis. The importance of the ageing and the role of the atmospheric CO 2 on the nature and morphology of the corrosion products are discussed. The corrosion resistance of Zn–Mg alloy is correlated with the stabilization of simonkolleite against its transformation into smithsonite, hydrozincite, and zincite during ageing cycles in presence of CO 2. The stabilization appears to be due to the preferential formation of magnesium carbonates. Thermodynamic modeling and titrometric analysis demonstrate that Mg 2+ enhances simonkolleite during dry–wet cycling by (1) removing carbonate from the environment and thereby limiting of the transformation of simonkolleite into zincite, smithsonite, and hydrozincite and by (2) buffering the pH of the electrolyte around 10.2 due to the precipitation of Mg(OH) 2 preventing the dissolution of zinc based corrosion products into soluble hydroxide complexes.

Corrosion resistance of Zn–Co–Fe alloy coatings on high strength steel

The corrosion properties of electrodeposited zinc–cobalt–iron (Zn–Co–Fe) alloys (up to 40 wt.% Co and 1 wt.% Fe) on steel were studied by using various electrochemical techniques and compared with zinc (Zn) and cadmium (Cd) coatings in 3.5% NaCl solution. It was found that with an increase in Co content in the coating the open circuit potential (OCP) became more positive than that of the zinc coating. For Co contents higher than 30 wt.% the OCP shifted close to that of Cd, but still remained electronegative to the steel substrate. Zn–Co–Fe coatings with ≥ 30 wt.% Co + 1 wt.% Fe are nano-crystalline in nature and show superior corrosion resistance as compared to the Zn, low Co content Zn–Co–Fe and Cd coatings. During longer immersion, Zn–Co–Fe alloys with 34–40 wt.% Co became more noble to steel due to dezincification of the surface but the corresponding corrosion current density decreased. The corrosion resistance determined by the electrochemical techniques are confirmed by salt spray testing showing the superior corrosion resistance for Cd and high Co content Zn–Co–Fe alloys and poor performance of alloys with lower Co contents and pure Zn.

Effect of substrate heat treatment on morphology and corrosion resistance of Zn–Mn phosphate coating

Phosphate coatings are one of the ancient methods for improvement the corrosion resistance of steels. Many researchers have investigated the effect of process parameters on the structure and properties of these coatings, but the influence of substrate microstructure has been less studied. In this article, the effect of heat treatment of AISI 4130 steel on the surface morphology and corrosion resistance of Zn–Mn phosphate coating has been investigated. The steel substrate was subjected to different heat treatments before coating application. X-ray diffraction, optical and scanning electron microscopy were used for phase and microstructure analysis respectively. The corrosion resistance of the coatings was studied by means of salt spray, potentiodynamic polarisation and electrochemical impedance spectroscopy methods. The results showed that the corrosion resistance of phosphate coatings is greatly influenced by substrate microstructure. The finer the steel microstructure, the more condensed becomes the coating morphology and the better its corrosion resistance.

Electrodeposition and characterisation of Zn, Zn–Fe and Zn–Fe–Ni coatings in presence of gelatin as additive

Studies of the factors that enhance the corrosion behaviour of electrodeposited Zn, Zn–Fe and Zn-Fe-Ni coatings are highly relevant due to the considerable industrial use of these alloys as protective coatings for steel substrates. This paper reports on the corrosion behaviour and structure of Zn, Zn–Fe and Zn–Fe–Ni coatings deposited from an acid citrate solution studied in the absence and in the presence of gelatin as additive. The presence of Fe or Fe–Ni in the Zn matrix increases Zn coating corrosion resistance. The presence of gelatin in the plating bath increases the corrosion resistance of Zn, Zn–Fe and Zn–Fe–Ni coatings.

Deposition of cobalt rich Zn–Co alloy coatings of high corrosion resistance

Cobalt rich Zn–Co alloy coatings were electrodeposited from an alkaline electrolyte under equilibrium (not anomalous) codeposition conditions. The main part of the study deals with Zn–15Co alloy coatings possessing the best appearance. X-ray diffraction (XRD) measurements ascertained that the alloy structure consisted of a single gamma phase Co5Zn21. Electrochemical impedance spectroscopy (EIS) showed that the corrosion resistance of Zn–15Co at the initial corrosion stages was about three times as high as that of pure Zn and comparable to the corrosion resistance of pure Co. Testing in salt mist revealed the corrosion resistance of Zn–15Co under these conditions to be about three times higher than that of cobalt lean Zn–1Co alloy. The morphological properties and the chemical composition of corrosion products were studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Zinc was found to be in an oxidised state within a corrosion product layer, whereas cobalt was not oxidised. Based on this, development of some cobalt enriched protective layer during corrosion was assumed. The results demonstrated that Zn–15Co coatings, owing to their high corrosion resistance, may be considered as a replacement for chromated coatings (Zn and Co lean alloys) or Zn–Ni.

Pulse plating effect on composition and corrosion properties of zinc alloy coatings

The influence of pulse plating parameters on the chemical and phase composition, surface topography and corrosion resistance of Zn–Ni (∼12%Ni), Zn–Co (<1%Co) and Zn–Fe (<1%Fe) alloy coatings has been studied. Pulse plating of Zn alloys resulted in different outcomes for Zn–Ni and low alloyed (Zn–Co and Zn–Fe) coatings. Non-stationary crystallisation resulted in formation of single phase γ-Zn21Ni5 coatings, while two different phases of Zn were produced during Zn–Co and Zn–Fe deposition. Pulse plated Zn–Ni coatings possessed from two to three times smaller grain size and at the same time, corrosion currents half those of the direct current deposits. However, the reduction in the grain size of Zn–Co and Zn–Fe coatings was not accompanied by any reduction in the corrosion current of these alloys. It is suggested that the dual phase formation was the principal reason why pulse plated low alloyed Zn coatings did not exhibit improvement in their corrosion resistance.

The Role of Macrostructural and Microstructural Morphologies on the Corrosion Resistance of Zn and a Zn-4% Al Alloy

Different structural morphologies may develop due to a wide range of the operational conditions that may exist during casting. It is well known that corrosion resistance and mechanical properties depend on solidification structures. The aim of this study is to investigate both the influence of columnar and equiaxed structures of zinc as-cast samples and of dendritic microstructural array of a Zn-Al alloy on the corrosion resistance. In order to obtain columnar and equiaxed structures, both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed using electrochemical impedance spectroscopy and Tafel extrapolation. Corrosion tests were conducted in a 3% (vol.) NaCl solution at room temperature. It was found that coarser macrostructures tend to provide higher corrosion resistance for both columnar and equiaxed morphologies. On the other hand, finer secondary dendrite arm spacing is conducive to a better corrosion behavior of a Zn-4% Al alloy.

Silicone resin to improve corrosion resistance of Zn and ZnFe coated steel

Chromatation pre-treatments have been widely used to improve galvanized steel corrosion resistance. However, due to the high toxicity of chromate ions, chromatation pre-treatments tend to be banned and, in last years, alternative coating systems are under investigation. Recently, polysiloxanes have been developed for application as coatings. Among them, and due to their specific properties, such as hardness, chemical resistance and hydrophobicity, silicone resins may be considered as promising substitutes for chromatation pre-treatments. In this work silicone films, obtained from the hydrolysis of a methoxy functional silicone reactive intermediate, were applied on galvanized steel and on steel electroplated with a ZnFe alloy. Electrochemical techniques were used to characterize the degradation behavior of the samples. These consisted on the monitoring of the open circuit potential (OCP), and on the potentiodynamic polarization of the samples, which was performed in a 3% NaCl aqueous solution. Additionally, electrochemical impedance spectroscopy (EIS) was used as a complementary technique for the evaluation of the corrosion mechanisms of the coating system. SEM and EDS were employed to inspect the surface of the samples before and after the electrochemical tests. EIS data was fitted to an equivalent circuit from which the electrochemical parameters were obtained. Results show the protective character of the resin films, when compared with uncovered specimens. The capacitance of the films increased with the immersion time, in accordance to the behavior expected for an organic film. The overall performance of the coating systems appears to be highly dependent on the type of metallic coating applied to the steel. During the first three days of immersion the coatings applied upon galvanized steel showed larger |Z| values when compared with those applied to the electroplated steel, indicating a superior corrosion resistance of the former. However, after that time, an abrupt drop of |Z| is observed in the film applied on galvanized steel. In comparison, the coating system involving ZnFe alloy evidences a better stability throughout the immersion time.

The role of macrostructural morphology and grain size on the corrosion resistance of Zn and Al castings

Different macrostructural morphologies and grain sizes may develop due to a wide range of the operational conditions that may exist during casting. It is well known that mechanical properties depend on solidification structures. Particularly, the literature presents relationship between the materials yield strength and grain size or ultimate tensile strength and primary and secondary dendrite arm spacings. It is also reported that the corrosion resistance has been shown to depend strongly on the structural morphology and chemical composition. The aim of this study is to investigate: (a) the influence of columnar and equiaxed structures on the corrosion resistance of aluminium and zinc as-cast samples, (b) the influence of longitudinal (columnar structure) and transversal (pseudo-equiaxed structure) sections of the same sample on the corrosion resistance for both metals and (c) the influence of equiaxed grain size of hot-dip galvanized steel sheets on the corrosion resistance. In order to obtain columnar and equiaxed structures both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed by electrochemical impedance spectroscopy technique and Tafel extrapolation method. Corrosion tests were conducted in a 3% NaCl solution at room temperature.

Relationship between microstructure and corrosion resistance in Zn–Al alloy coated galvanised steels

The effect of cooling rate on the coating microstructure and corrosion resistance of Zn - 4.5 wt-%Al alloy galvanising coatings applied to 0.7 mm gauge strip steel substrates is presented. Increasing the cooling rate by the use of a high powered air cooler does not significantly alter the volume fraction of primary zinc in the alloy coating layer, which remains at ~ 14 vol.-%. There is, however, a dramatic change in the number and size of dendrites with far greater numbers (changing from 270 to 1500 mm2) of smaller dendrites as the cooling rate is increased using cooler outputs of 55 and 100%, respectively. This corresponds to a change in the surface appearance of the material with the surface spangle (i.e. eutectic cell) size being reduced from 7.5 to 2.1 mm as the cooling rate is increased. Since the volume fraction of the primary zinc remains roughly constant and the dramatic change is in the size and numbers of dendrites, it appears that this phenomenon is related to increased nucleation of primary zinc. The scanning vibrating electrode technique (SVET) has been used to investigate the effects of these microstructural changes on the corrosion mechanism and kinetics. The SVET isocurrent contour maps show a dramatic reduction in corrosion intensity with increasing cooling rate. This is borne out by metal loss calculations, which show that the zinc losses from the coating are 2500, 1200 and 200 μg at cooler outputs 55, 80 and 100%, respectively. More significantly, the anodes display shorter lifetimes and lower intensities for the materials produced at the higher cooling rate. One quarter of the anodes in the specimen cooled at 55% remained active throughout the 24 h immersion period compared with 4% of the anodes in the specimen cooled at 100%. The spangle size is directly related to the cooling rate and the microstructure and can be related directly to the cut edge corrosion resistance as determined by SVET.