3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel

Recently, experimental work has been reported that demonstrates the effects of microstructural variations within Zn-Al Galfan type coatings on the corrosion behaviour of cut-edge material, i.e. those cases where both the underlying steel and the organic coated Galfan layer are simultaneously exposed to a corrosive environment [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corros. Sci. 47 (11) (2005) 2740-2759]. In this paper a finite difference numerical model of localized corrosion has been applied in an attempt to simulate this type of corrosion. Results from the model are compared to experimental observations.     

Zinc‐Aluminium‐Coatings as Corrosion Protection for Steel

An analysis was made to investigate the influence of the alloying of sprayed coatings on the corrosion performance in climates of practical relevance and to evaluate the optimised alloy compositions. Furthermore, the mechanism of corrosion and coating formation was evaluated. The protective effect increased up to an aluminium content of 15 and 22 wt%, respectively, with corrosion rates below 1 µm per year. Electrochemical investigations found a distinctive decrease in the cathodic reaction together with an accumulation of aluminium in per cent and a depletion of zinc. Due to the increasing concentration of aluminium within the layer and the correlated presence of inhibitive corrosion products, the cathodic reaction and therefore the corrosion of the sprayed zinc‐aluminium coatings can be increasingly eliminated.     

Steel cathodic protection afforded by zinc, aluminium and zinc/aluminium alloy coatings in the atmosphere

Zinc has traditionally been the metallic material most widely used to protect steel against atmospheric corrosion due to its ability to afford cathodic protection to steel in all types of natural atmospheres. In recent decades, aluminium and zinc/aluminium alloy coatings have been used instead of zinc in certain atmospheric applications. Although these coatings present some advantages over zinc, they are not able to cathodically protect steel substrates in all types of natural atmospheres. The present paper assesses the cathodic protection afforded by Al (flame spraying), Al/13 Si (hot dipping), 55Al/Zn (hot dipping), Zn/15Al (flame spraying), Zn/5Al (hot dipping), Zn (hot dipping), Zn (discontinuous hot dipping) and Zn (electroplating). Aluminium and aluminium-rich alloy coatings (55%Al/Zn) provide cathodic protection to the steel substrate only in atmospheres that are highly contaminated with chloride ions (>100 mg Cl⁻ m⁻² day⁻¹) where these coatings become active.     

Korrosionsverhalten von feuerverzinktem Stahl in erwärmten Wasser1

Corrosion behaviour of hot galvanized steel in warm water Pipe specimens made of commercial grade galvanized steel and zinc have been tested in comparison to pipe specimens having modified zinc coatings. The specimens were corroded in 11 test lines with warm water at approx. 60 °C and cold water with continuous and intermittent flow. Dortmund town water with polyphosphate and orthophosphate additions in individual test lines was used for the tests. One test line was operated with warm water from a water heater with cathodic vessel protection according to the Guldager method. The investigation included visual assessment, determination of mass loss, and electrochemical measurements. The main results were as follows: 1. The localised corrosion tendency is not determined by the amount of potential ennoblement, but rather by the inhibition of the cathode reaction on the surface layer formed, which can be read off the cathodic current density vs. potential curve. The inhibition of the cathode reaction can be affected both by variations of the material and by variation of the water quality. 2. The tested materials with modified coatings featured throughout a poorer corrosion behaviour in warm water than commercial zinc coatings according to DIN 2444. 3. The phosphates used for the investigation induce an inhibition of the cathode reaction in zinc. Zinc-iron alloy phases in warm water fail to ensure an appropriate effect. A treatment of the water according to the Guldager process ensures a strong inhibition of the cathode reaction both with zinc and especially with zinc-iron alloy phases and induces an important improvement of the corrosion behaviour of galvanized steel in warm water.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an “alkalisation”, “passivation”, “poisoning” effect or “shortcut” effect.     

Zur Korrosion metallischer Werkstoffe in Tetrachlorkohlenstoff

Corrosion of metals in carbon tetrachloride Nickel, nickel alloys, titanium, the titanium-palladium alloy, carbon steel, stainless steel, zinc, aluminium und lead were immersed in the following boiling media: carbon tetrachloride dried and saturated with water and two phase mixtures of carbon tetrachloride and water. Titanium, the titanium alloy and nickel alloys of type C-4, G and 825 are high-corrosion-resistant, the alloy B-2 were only slightly corroded. Nickel, alloy 600, zinc and lead were corroded in the boiling two phase mixture. The weight loss of the stainless steel is very low, but the sensitivity of these steel grades to pitting and/or stress corrosion cracking have to be taken into account. Carbon steel and aluminium corroded heavily in contact with the boiling mixture. The composition of the gaseous phase corrosion products shows different corrosion mechanism.     

Corrosion resistance of painted zinc alloy coated steels

Organic coating in combination with sacrificial metal coating is the most popular method of protecting steel strips against atmospheric corrosion. Experiences over the years have proven that such duplex coating systems are best suited for the coil industry for the long term corrosion protection of steel. The excellent corrosion resistance of such systems has been attributed to the synergy between the cathodic protection provided by the sacrificial coating of zinc alloys and the combined barrier resistance of the metal and organic coatings. Traditionally continuously hot dip zinc-coated steels are used for such applications. However, off late the quest for further extending the longevity of the coil coatings has led to the replacement of the zinc coating with a host of other hot dip zinc–aluminium alloy coatings such as Galvalume^®, Galfan^®, ZAM^®, SuperDyma^®, etc. Each of these metal coatings has its own unique metallurgical features in terms of flexibility, bonding, microstructure and electrochemical characteristics which may significantly influence the performance of the organic coatings applied over it. This paper looks into the various aspects of these features of the hot dip coatings on the corrosion performance of the pre-painted steel strips. For simplicity only polyester paint system, the work horse of the coil industry, is considered.     

Corrosion and corrosion test methods of zinc coated steel sheets on automobiles

The effect of zinc and zinc alloy coated steel sheets on perforation corrosion in actual automobiles and the relevant accelerated corrosion test methods were studied. The main factor affecting corrosion in the crevice of lapped panels was the coating weights of zinc and zinc alloys rather than the type of coating. Perforation corrosion process of galvanized steel in the crevice of lapped portion in automobiles was divided in four stages. Based on the analysis of corrosion in actual automobiles, a Perforation Corrosion Index, PCI for lapped steel panels was proposed. Assuming PCI for various accelerated corrosion test methods, the corrosion resistance of various types of coated steel sheets in actual environments was evaluated.     

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.     

The Corrosion Behavior of Cold Sprayed Zinc Coatings on Mild Steel Substrate

Zinc and its alloy coatings have been used extensively for the cathodic protection of steel. Zinc coating corrodes in preference to the steel substrate due to its negative corrosion potential. Numerous studies have been conducted on the corrosion behavior of zinc and its alloy coatings deposited using several techniques viz., hot dip galvanizing, electrodeposition, metalizing or thermal spray etc. Cold spray is an emerging low temperature variant of thermal spray family which enables deposition of thick, dense, and pure coatings at a rapid rate with an added advantage of on-site coating of steel structures. In the present study, the corrosion characteristics of cold sprayed zinc coatings have been investigated for the first time. In addition, the influence of heat treatment of zinc coating at a temperature of 150 °C on its corrosion behavior has also been addressed.     

Korrosionsverhalten gespritzter Zink‐Aluminium‐Überzüge in Kurzzeitbewitterungstests

Corrosion behaviour of thermally sprayed zinc‐aluminium‐coatings in short‐term corrosion tests By alloying aluminium to zinc the corrosion resistance can be increased in a multiplicity of media. Thermally sprayed ZnAl 15‐coatings show a higher corrosion resistance than thermally sprayed zinc‐ or hot dip galvanized coatings. This paper presents the results of short term corrosion tests (salt spray test, Kesternich test) with sprayed ZnAl‐coatings with an aluminium content of 2 to 50 M.‐%. The investigations verify very clearly the influence of aluminium on the corrosion behaviour. Depending on the aluminium content the corrosion resistance shows a maximium with a medium content of about 15 or 22 M.‐%.     

Corrosion resistance of steel with hot‐dipped galvanized zinc and zinc alloy coatings in seawater

Hot‐dipped galvanized zinc and zinc alloy coatings were used as protective metallic coatings for steel structures in seawater in China. Corrosion of the two coatings immersed in sea water in Qingdao and Xiamen for 6 years were introduced and analyzed, which provides a basis for further development and applications of these coatings in China. Tests proved that the anti‐corrosion performance of the hot‐dipped low alloy zinc coatings (aluminum content less than or equal to 10 wt%) is equal to or even lower than that of the pure zinc sheet, while the performance of the hot‐dipped high alloy zinc coatings is higher than that of the pure zinc sheet. The hot‐dipped high alloy zinc coatings can be further developed for optimal performance in the future.     

Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution

This work is focused on the role of intermetallics in pitting corrosion of Al2219 alloy. Second phase particles were characterized by AES, SAM and EDX. Their behaviour in a solution of NaCl was investigated as a function of exposure time. The results confirmed the cathodic nature of the intermetallics with respect to the aluminium matrix. Corrosion products rich in aluminium and oxygen were found to progressively accumulate around the particles and iron was dissolved from the intermetallic, followed by back-deposition. Copper and manganese did not show any major activity. After 32 h of exposure the larger intermetallics were completely covered.     

The influence of the cathodic process on the interpretation of electrochemical noise signals arising from pitting corrosion of stainless steels

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.     

Influence of chloride ion concentration and temperature on the corrosion of Mg-Al alloys in salt fog

The influence of temperature and chloride concentration on the corrosion behaviour of Mg-Al alloys exposed to salt fog was evaluated. Corrosion attack increased with decreasing aluminium content in the alloy and increasing Cl⁻ concentration and temperature. The effect of Al-Mn inclusions, which revealed several stoichiometries and were up to 300 mV more noble than the magnesium matrix, was only noticeable in the early stages of corrosion of the AZ31 alloy. Aluminium segregation and β-phase distribution were the main controlling factors for the AZ80 and AZ91D alloys, the latter being more susceptible to variations in the saline concentration.     

Effect of cathodic protection on corrosion of pipeline steel under disbonded coating

In this work, a test rig was developed to study the effect of cathodic protection (CP) on corrosion of X70 pipeline steel in the crevice area under disbonded coating through the measurements of local potential, solution pH and dissolved oxygen concentration. Results demonstrated that, in the early stage of corrosion of steel, CP cannot reach the crevice bottom to protect steel from corrosion due to the geometrical limitation. Corrosion of steel occurs preferentially inside crevice due to a separation of anodic and cathodic reaction with the depletion of dissolved oxygen in the crevice solution. The main role of CP in mitigation of sequential corrosion of steel in crevice under disbonded coating is to enhance the local solution alkalinity. With the increase of distance from the open holiday, a high cathodic polarization is required to achieve appropriate CP level at crevice bottom. A potential difference always exists between the open holiday area and inside crevice, reducing the CP effectiveness.     

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.     

Das Korrosionsverhalten von Eisen,Zink, Aluminium und Chromnickelstahl im System Methanol-Wasser-Chlorwasserstoff

Corrosion behaviour of iron, zinc, aluminium and chromium nickel steel in the system methanol/water/hydrogen chloride The influence of water on the corrosion behaviour of pure iron, zinc, aluminium and normal 18 9 CrNi steel in hydrogen chloride containing methanol has been studied. The immersion tests and the tracing of potential dynamic current density potential curves were carried out at 20°C, oxygen being excluded. Even small quantities of water may give rise to a considerable decrease of corrosion rates. This effect is attributed in the case of iron, zinc and chromium nickel steel to increased hydrogen overvoltage, which exhibits a sudden jump between 0 and 15 Vol.-% water. Heterogenous cathodic limiting reaction currents are observed on iron. In the case of aluminium the change in corrosion rates toward lower values is caused by the inhibited anodic partial reaction. Addition of 5 Vol.-% water gives rise to a displacement of the pitting potential by approx. 400 mV in the positive direction, accompanied by a decreasing corrosion rate by about three orders. Anodic polarization of chromium nickel steel reveals that this steel can be passivated only beyond a critical water content of approx. 50 Vol.-%. A further increase of the water content gives rise to a sudden decrease of the critical passivation current density and the passivity current density. Simultaneously, the passivation potential moves toward more negative values.     

Corrosion properties of laser beam joints of aluminium with zinc-coated steel

This paper presents results of accelerated corrosion tests in a salt spray chamber as well as microelectrochemical measurements of thermally joint steel-aluminium mixed materials. The focus was set on analysing the corrosion behaviour of the different metallic materials (brazed seam, intermetallic phases, aluminium and steel sheet) in or within the vicinity of the brazed seam.Both corrosion tests show that the joining zone has the most negative corrosion potential and is the first to corrode. The degree of corrosive deterioration depends on the cathodic behaviour of the adjacent metal. Next to effective cathodes, such as steel or Fe-containing intermetallics, the attack is the most. However, contact to an aluminium alloy (AA6016) with its insulating oxide layer does not affect the corrosion properties of the respective filler materials.