The atmospheric corrosion resistance of electrodeposited tin-cadmium and tin-zinc coatings on steel

Abstract

A study has been made of the corrosion-resisting properties of tin-cadmium and tin-zinc coatings on steel in which both laminar and alloy coatings have been tested using atmospheric exposure and accelerated tests. It is concluded that electrodeposited coatings of zinc on a tin substrate or zinc alone possess a greater resistance to atmospheric corrosion in an industrial atmosphere than equivalent coatings of cadmium on tin or tin on cadmium, or tin-zinc alloy coatings. The corrosion resistance of tin-zinc laminar coatings depends primarily on the amount of zinc present. Alloy coatings of tin-zinc and tin-cadmium generally possess similar corrosion resistance to those of the tin and cadmium laminar coatings. Under simulated marine conditions (A.R.E. Salt Droplet test) the corrosion resistance of coatings containing cadmium is superior to that of corresponding coatings containing zinc.     

Electroplating on Aluminium

Coatings containing tin and cadmium, produced either as a layered electrodeposit of the two metals or by electrodepositing alloys from sulphate or fluosilicate baths, have been subjected to corrosion tests in comparison with other coatings.

Coatings of tin-cadmium alloy or of cadmium deposited over an equal thickness of tin have been found to have outstanding durability in laboratory salt-spray tests and, in all environments, are as well able as cadmium coatings to prevent rusting at pores when freshly exposed to corrosion. The alloy coatings have an advantage over cadmium in that they are less affected by the organic vapours emitted by some insulating materials. It has also been shown, by other investigators, that the alloy coatings have useful anti-galling properties.

Protection of steel exposed to the weather by the sea has lasted longer for some alloy coatings than it has for cadmium coatings of the same thickness, but it is doubtful whether this superiority can be obtained consistently. Compared with cadmium coatings, alloy coatings were no better in continuous or intermittent immersion in the sea and were inferior in exposure to inland urban atmospheres.

A tin undercoating 0·05mil? thick improved the performance of zinc coatings on flat plates in a marine atmosphere, but such an undercoating had no appreciable effect on the life of cadmium coatings in any type of exposure or of zinc coatings exposed inland.     

The corrosion resistance of electrodeposited zinc-nickel alloy coatings

The corrosion behaviour of electrodeposited zinc-nickel (Zn-Ni) alloy coatings has been studied in aqueous chloride environments. The corrosion rates of detached zinc alloys containing up to 25% Ni by weight were determined using linear polarisation techniques. The corrosion rate of Zn-Ni alloys was found to decrease with increasing Ni content over the compositional range studied. Galvanic corrosion measurements have indicated, however, that Zn-Ni alloy coatings become less sacrificial toward steel as the Ni content is increased. These results are used to interpret the corrosion behaviour of electroplated steel in a neutral salt fog environment.     

Trivalent passivation of plated zinc and zinc alloys—alternatives to hexavalent based systems

Hexavalent chrome passivates have been used for improving the corrosion resistance of sacrificial zinc and zinc alloy coatings on ferrous substrates for many years now. However, forthcoming legislation and corporate policies are beginning to curtail the use of hexavalent chrome compounds and replacements are actively being sought.

Comparative corrosion resistance data using passivation systems based on trivalent chrome is presented here. This data demonstrates at least the same performance to that obtained by hexavalent systems currently used and also shows that the corrosion resistance of the trivalent chrome free passivates does not significantly diminish after thermal shock treatment. This makes it particularly suitable for components that are subjected to high ambient temperatures. No changes to the dimensional characteristics of the components occur. Recommended application areas include the engine compartment, brake components, fasteners of all sizes and fluid system components.

When used in conjunction with recommended final finishes, these hexavalent chrome-free (Cr6⁺) passivates have the potential to replace more expensive alloy or dip spin type 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.     

Atmospheric Exposure Trials of Electrodeposited Iron-Zinc Alloy Coatings

Abstract

Iron-zinc alloy coatings applied by electrodeposition have been tested for their protective quality on steel exposed to an industrial atmosphere. A coating consisting of iron 40% and zinc 60% was twice as corrosion-resistant as a zinc e1ectrodeposit of the same thickness. The improvement was more noticeable for the thicker coatings (1 or 2 mils) than for the thinner ones (0·1 or 0·5 mils). Coatings containlng from 50 to 80 % of zinc and the remainder iron were almost as good. The coatings developed a rather unattractive uniform dark grey-brown colour which was, however, clearly distinguishable from rusting of the steel. Resistance to damage, proteciion at scratches, the effect of superimposed paint films and the performance of the coatings in other atmospheres will need checking before they can be used commercially.     

Microbially influenced corrosion and inhibition of nickel–zinc and nickel–copper coatings by Pseudomonas aeruginosa

The present study describes the effect of Pseudomonas aeruginosa on the corrosion rate of nickel–zinc and nickel–copper alloy coatings. The presence of bacteria was associated with decreases in R_ct values, suggesting that P. aeruginosa promoted the corrosion of nickel–copper alloy coatings. However, R_ct values of nickel–zinc coatings increased in response to bacterial inoculation, corresponding to a decrease in corrosion rate for nickel–zinc alloy coatings. Our results suggest that the activity of P. aeruginosa facilitated the corrosion of nickel–copper alloy, while serving a protective function for the nickel–zinc alloy.     

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 of furnace wall materials in waste-wood fired power plant

Abstract

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.     

Role of cadmium on corrosion resistance of Zn-Ni alloy coatings

Cadmium (Cd) catalyzed Zn-Ni alloy plating has been accomplished galvanostatically on mild steel (MS) using gelatin and glycerol as additives. The effect of addition of Cd into Zn-Ni bath has been examined in terms of nickel (Ni) content and corrosion resistance of Zn-Ni-Cd ternary alloy coatings. The process and product of electrolysis under different concentrations of additives and Cd have been investigated by cyclic voltammetry (CV). The effects of current density (c.d.) on Ni content of the alloy have been studied by spectrophotometric method, supported by EDX analysis. The deposition has been carried out under different concentrations of Cd ranging from 0.004 to 0.1 M. The corrosion rates (CR) of Zn-Ni alloy coatings have been found to decrease drastically with addition of Cd. It has been also revealed that the CR of binary Zn-Ni alloy coatings decreased with the increase of Cd concentration only up to a certain optimal concentration, i.e., up to 0.02 M, and then remained unchanged. An effort to change the anomalous type of codeposition into normal one by changing the molar ratios of the metal ions, i.e. [Cd²⁺]/[Ni²⁺] as 0.01, 0.05 and 0.25 has remained futile. CV study demonstrated an important role of Cd in mutual depositions of Zn²⁺ and Ni²⁺ ions by its preferential adsorption, thus leading to the increased Ni content of the alloy. The bath composition and operating parameters have been optimized for deposition of bright and uniform Zn-Ni-Cd alloy coatings. Changes in the surface morphology and phase structure of Zn-Ni alloy coatings due to addition of Cd has been confirmed by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) study respectively. Experimental investigations so as to identify the role of Cd in codeposition Zn-Ni alloy coatings have been carried out and the results are discussed.     

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.     

An assessment of climate change effects on atmospheric corrosion rates of steel structures

Abstract

This paper presents a discussion on assessing the potential impacts of climate change on the atmospheric corrosion rates of exposed steel structures. The effects on atmospheric corrosion due to changes in the environmental temperature, carbon dioxide, relative humidity, wind, rainfall and pollution are considered. The limitations and complexities of these assessments are discussed. To demonstrate the use and limitations of this science to evaluate effects related to climate change, a model developed in Australia to predict corrosion is combined with climate change models to project the change in the corrosion rates of steel components and protective zinc coatings in constructions. The method is applied to constructions located along the coastal areas of two Australian cities: Melbourne and Brisbane. These assessments are made using the A1FI scenario, the highest emission scenario defined by the Intergovernmental Panel on Climate Change, applied to nine general circulation models. The projected changes in corrosion rates were found to be an increase of ～14% for both zinc and steel in Brisbane and a decrease of ～14% for steel and 9% for zinc in Melbourne. It was also found that the uncertainties associated with the climate change models were small compared to those involved in modelling corrosion for engineering purposes.     

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.     

Electrodeposition and properties of Zn,Zn–Ni,Zn–Fe and Zn–Fe–Ni alloys from acidic chloride–sulphate electrolytes

Abstract

Zn, Zn–Ni, Zn–Fe and Zn–Fe–Ni films have been deposited by electrochemical deposition technique onto steel plate substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for the application as new environmentally friendly sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied using XRD and SEM techniques. Cyclic voltammetry of the individual metals was performed to help understand the electroplating process of the films. Grain sizes of the films were calculated using Scherrer's formula. Partial substitution of Zn to Fe and Ni leads to an improvement in the corrosion resistance. Compared with other zinc alloys, the Zn–Ni alloy deposit was the noblest.     

Combination of zinc alloy coating with thin plasma polymer films for novel corrosion protective systems on coated steel

Steel sheet used in automotive applications has to be corrosion protected effectively, which is usually realized by zinc or zinc alloy coatings with a thickness range of 5–10 μm. Steel sheet for areas of a car body which are exceptionally stressed by corrosion, e.g. cavity flanges or joints, may be protected additionally by a thin weldable organic coating with a thickness of 2–4 μm. A very promising approach to a significantly reduced use of resources is the combination of zinc alloy coatings with thin plasma polymer films deposited by means of plasma-enhanced chemical vapour deposition (PECVD). Such plasma polymer films of just a few 100 nm thickness show excellent barrier and adhesion properties as well as a high mechanical stability.Within this work thin plasma polymer films were deposited on zinc alloy coated steel substrates using the strip hollow cathode (SHC) method, which was modified for application on grounded substrates. A pulsed DC glow discharge in a mixture of argon and an organosilane precursor was used for the deposition of films with a thickness of 100–500 nm.The chemical compositions of the coatings were determined by means of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The morphologies of the coating systems were studied by means of scanning electron microscopy. The performance of the coating systems has been studied in different specific tests of corrosion and processing behaviour. The investigated coating systems show a corrosion resistance comparable to reference samples of electro-galvanised steel sheet with additional organic coating even with a coating thickness less of half of the reference samples.     

Corrosion of zinc–magnesium coatings: Mechanism of paint delamination

Due to their promising corrosion properties, metallic coatings containing magnesium are currently widely investigated for use as protective coatings for steel sheet. Particularly, alloying zinc coatings with magnesium results in a remarkable improvement of the corrosion resistance of the painted system. While some aspects of this improvement have been understood, the progress of the corrosive degradation of the alloy coating/paint interface has not been reported in detail. In this paper, the delamination of a model polymer from the intermetallic MgZn₂ is described and a degradation mechanism proposed. Aspects for the design of stable interfaces are discussed.     

镀锌钢构架在北美住宅环境中的耐蚀性能

钢结构住宅是未来住宅建筑的发展方向之一,而住宅钢结构防腐蚀寿命也成为钢结构住宅推广的关键因素之一。国际铅锌研究组织项目"住宅镀锌钢构架在实际住宅环境下的耐蚀性研究"通过选取北美4种典型的住宅环境,对锌及锌合金镀层钢铁试样进行了连续7年的暴露试验,并对镀锌钢构架在不同住宅环境中的腐蚀速率进行了评估。公布的研究结果表明,镀层质量为200~270 g/m2的镀锌钢以及质量相当的锌合金镀层钢试样在4种环境下腐蚀速率均非常小,以所有试样中的最大腐蚀速率来评估镀锌及锌合金钢试样在4种环境下的使用寿命可达258~964年,平均574年。在住宅设计使用年限内,采用镀锌钢结构不会因腐蚀而使得钢结构性能降低。镀锌钢可以作为一种可持续性的建筑材料使用。     

Papers to be presented at Eurocor '77

Abstract

Despite numerous studies on atmospheric corrosion of copper and copper based alloys, the corrosion induced release processes of individual alloy constituents suffer from significant knowledge gaps. This investigation comprises metal release rate measurements of copper, zinc and tin from some copper based alloys including brass (20 wt-%Zn) and bronze (6 wt-%Sn), and their pure alloying metals, copper, zinc and tin. Data have been generated during a 2·5 year urban field exposure in Stockholm, Sweden and parallel laboratory investigations in a specially designed rain chamber using artificial rain. Brass shows significantly lower annual release rates of both copper and zinc compared to pure metal sheets of its alloy constituents. Zinc is preferentially released compared to copper. Dezincification of brass occurs both at field and laboratory conditions, a process influenced by rain characteristics. Alloying with tin does not largely reduce the release rate of copper from bronze compared to pure copper. No measurable amount of tin is released from the bronze surface.     

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291-2303], first-order model used to simulate the localized degradation experienced in Zn-4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [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, Corrosion Science 47 (2005) 2740-2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.     

A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.