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.     

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.     

Investigations on the Electro-Deposition of Silver,with Special Reference to the use of Sodium Cyanide

The results of an investigation on the deposition of tin-zinc alloys, ranging in composition from 92/8 to 28/72, are discussed, and instructions are given for the deposition of the alloy containing 78% of tin, which appears to be the most serviceable. The electrolyte contains sodium stannate, zinc cyanide, sodium hydroxide, and sodium cyanide and is operated at 70°C., using tin-zinc alloy anodes of approximately the same composition as the deposit. A feature of the process is polarisation of the anodes so as to ensure dissolution of the tin as stannate. The influence of changes in the composition of the electrolyte, in cathode current density, and in temperature on the composition of the deposit have been examined and measurements have been made of the effect of alteration in the working conditions on anode and cathode efficiency. Chemical methods for the analytical control of the plating bath and alloy deposit are given.     

Synopses of papers from other institute journals

Abstract

The corrosion behaviour of a range of readily available commercial copper based alloys was examined in three accelerated atmospheric corrosion tests: acidified salt spray, neutral salt spray at 80°C, and the GM (salt spray with thermal cycling) test. The samples exposed to the GM test evinced very little corrosion. During exposure, the weight of all the samples fluctuated in a random fashion, indicating that the corrosion products were only partially adherent and became detached from the surface at random times. The sequence of corrosion product formation on the specimen surfaces during exposure to the neutral salt spray test was very similar to that observed on coppers exposed to the atmosphere, indicating that this test can be used to give accelerated indications concerning atmospheric corrosion in humid marine regions. Four 24 h cycles of salt spray exposure are approximately equivalent to 1 year's exposure to the atmosphere. Alloying copper with phosphorus, tin, zinc, or silicon does not improve atmospheric corrosion resistance. The alloy containing 30%Zn exhibited dezincijication. The nickel silvers exbibited corrosion rates in the neutral salt spray at 80°C that were lower by a factor of at least 13 than those of the high purity coppers. For six of the alloys studied, the as received surface condition correlated with a slightly, but statistically significantly, higher corrosion rate, attributed to residual contaminants from rolling.     

Galvanic corrosion of aluminium alloy members of bridge guiderails under severe atmospheric exposure conditions

ABSTRACT

Aluminium alloys are nowadays preferred as materials for bridge guiderails especially for bridges connecting oceanic islands or spanning inlets due to their inherent corrosion resistance. But because of the limited mechanical strength of aluminium alloy, fasteners of guiderail members are made from steel materials. It has been found that contact between bare steel fasteners and aluminium alloy members can cause galvanic corrosion in the aluminium alloy. Research was carried out to investigate the capability of different surface treatments on fasteners and aluminium alloy members to inhibit galvanic corrosion under atmospheric exposure for periods of one and three years. It was found, among other results, that stainless steel fasteners treated with zinc flake coating were the most effective inhibitors of galvanic corrosion on aluminium alloy members.     

Corrosion behaviour of thermal sprayed nitinol coatings

NiTi alloy is here investigated as an alternative coating to stainless steel since it is considered to possess good corrosion properties. Three different thermal spray techniques (high velocity oxy-fuel -HVOF-, vacuum plasma spray -VPS- and atmospheric plasma spray quenching -APS+Q-) have been used for building the coatings, and electrochemical tests have been carried out for corrosion evaluation. Open-circuit tests have revealed that the VPS-coating shows fairly good corrosion resistance, both in the as-sprayed and polished forms. The HVOF coatings however, showed a strong dependence on the surface conditions and APS+Q is dominated by electrolyte penetration through coating cracks, thus exhibiting a higher i_corr.     

The Structure and Protective Properties of Zinc and Nickel Alternate Electrodeposits

A NEW TYPE of coatings consisting of a large numberof laminar deposits has been the keen subject of muchresearch,because these layered-structure coatingspossess improved properties or novel phenomenon suchas increased mechanical strength,micro-hardness,giantmagnetoresistance and corrosion resistance[1"6'.Thesecoating systems with individual layers making upoverall structure are also called compositionallymodulated multilayer(CMM)coatings'3"5'.In view ofthe plating historical use of surface…     

Corrosion resistance of hot‐dipped zinc and zinc alloy coated sheet steels under offshore atmospheric environment

The application of hot‐dipped zinc and zinc‐aluminum alloy coatings were introduced. Exposure tests of the steels with these coatings were conducted in the offshore atmosphere in Qingdao and Xiamen for 12 years separately. Effects of the coating thickness, alloy composition and atmospheric environment on the corrosion performance were studied. Results of the onsite exposure tests were compared with the results of a previous indoor salt spray accelerated corrosion tests. The study supports that zinc‐aluminum alloy coatings are useful in providing better corrosion resistance and can be further developed for future applications.     

Zinc whisker growth from electroplated finishes – a review

Abstract

Electroplated zinc finishes have been associated with the electronics industry for many years as a result of their excellent corrosion resistance and relatively low cost. They are normally applied onto ferrous products to provide corrosion protection in a range of different environments. However, the formation of spontaneously grown whiskers on zinc-electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long life electrical and electronic equipment. The growth of zinc whiskers has been identified as the cause of some electrical and electronic failures in telecommunications and aerospace-based applications, with consequences ranging from mild inconvenience to complete system failures. Investigators have been striving to address the problems induced by whisker growth since 1940s. However, most research effort has been focused on tin whiskers, especially following European Union environmental legislation that restricted the use of lead (Pb), which when alloyed with tin (3–10% by weight) provided effective tin whisker mitigation. Compared with tin whisker research, much less attention has been paid to zinc whiskers. A number of mechanisms to explain zinc whisker growth have been proposed, but none of them are widely accepted and some are in conflict with each other. The aim of this paper is to review the available literature in regard to zinc whiskers, to discuss the reported growth mechanisms, to evaluate the effect of deposition parameters and to explore potential mitigation methods. This paper presents a chronologically ordered review of zinc whisker-related studies from 1946 to 2013. Some important early research, which investigated whisker growth in tin and cadmium, as well as zinc, has also been included.     

Factors Influencing the Design of Automatic Plating Plant

A preliminary investigation has been made into the electrodeposition of tin-zinc alloy from cyanide-free alkaline solutions containing the trisodium salt of N-hydroxyethyl-ethylenediamine triacetic acid. The effect of the variables of the process on cathodic efficiencies, deposit composition and anodic efficiencies were studied and a bath of 40 litre capacity and capable of producing deposits containing 75% tin and 25 % of zinc was formulated and operated over several months. It has been found that the solutions are stable, and that the deposits obtained are similar to those obtained from the conventional stannate- cyanide solutions.     

The Adhesion of Electrodeposited Nickel To Brass

Although tin-zinc alloy plating has been practised commercially in England for several years, American knowledge of and interest in the process developed by the Tin Research Institute did not become appreciable until 1950; this delay was attributable partly to the continuing restrictions on tin use in America, and partly to normal reluctance on the part of industry to adopt a new process without extensive and time-consuming tests. The acute shortage of cadmium which developed late in 1950 provided the necessary stimulus, and the concurrent development of an all-potassium bath, which made the process directly competitive with cadmium-plating in speed of deposition, greatly aided in convincing the American electroplating industry that the tin-zinc process represented a worth-while innovation. Re-imposition of tin controls has somewhat clouded the issue but there is little doubt that the tin-zinc alloy deposit will find a place in industry. This paper presents operating data for the all-potassium bath and explains a discrepancy between English and American results using the sodium bath.     

Corrosion Resistance of Zinc-alloy Coated Steel in Construction Industry Environments

Considerable efforts are being made to improve the corrosion resistance of zinc coatings by alloying and application of compatible conversion coating treatments. These new systems have the potential for reliable and cost effective protective coatings on steel used in construction applications.

The present investigations have been concerned with: (i) development of a simple sensor that can be used to determine corrosion rates of zinc alloy coated steel in the macro/micro climates found in and around buildings and other structures.

(ii) determination of the compatibility and corrosion rates of zinc alloy coatings in contact with cementitious materials and preservative treated timber.

This paper describes the rationale for the investigations and the experimental techniques used, with a summary of the principle results and conclusions.     

Monitoring Atmospheric corrosion with electrochemical methods in laboratory environment

An atmospheric corrosion monitor (ACM) has been developed for monitoring atmospheric corrosion of Zn-coated iron and tin and SnPb coated copper in salt spray and hot steam atmosphere using the electrochemical technique of polarization resistance R_p. ACM for Sn and SnPb coatings is made using the printed circuits technology. The measurement of atmospheric corrosion using the R_p principle and the impulse galvanostatic method, is a novelty in the atmospheric corrosion studies. The method is fast, sensitive, non-destructive and suitable for all the anodic and cathodic coatings used on metals.     

Recent developments in the application of tin

As a commodity, tin has weathered a severe drop in its market value, and, at present, tin-producing countries are exporting more of the metal than was originally anticipated. Tin is utilized in a variety of significant applications, including: solder, where it has grown at the expense of lead; tinplate for food cans, where the battle between aluminum and steel is still being waged; coatings, where tin-zinc alloys are more corrosion resistant than zinc alone; and chemical production, where tin is used as a flame retardant and stabilizer in polymers. Fluctuations in tin prices would most adversely influence the fledgling chemical market, where the growth opportunities for tin diminish as the metal’s price escalates.     

The Atmospheric corrosion of statue bronzes exposed to SO2 and NO2

We report on the atmospheric corrosion of different cast statue bronzes in humid air containing ppb levels of SO₂ and SO₂+NO₂. In addition, copper, tin, zinc, and lead samples were studied in these environments. The samples were exposed to synthetic atmospheres with careful control of pollutant concentrations, relative humidity and flow conditions. Deposition of SO₂ was studied using on-line gas analysis. The weight gain was registered after four weeks exposure, and corrosion products were analysed by Electron Probe Micro Analyser (EPMA), X-ray diffraction (XRD), and Fourier Transform Infrared spectroscopy (FTIR). The synergistic effect of SO₂ and NO₂ was remarkable on all bronze materials examined. The weight gain was correlated to alloy composition. Thus, high zinc and low lead content resulted in the greatest weight gain, while high tin content favoured a low weight increase. Lead exhibited a rapid deposition of SO₂ followed by zinc and copper, while tin was unreactive towards SO₂. There was no measurable indication that microstructure influenced corrosion. The corrosion product morphology found in SO₂+NO₂ environment indicated a localised type of attack. The anodic sites were covered by a tin-rich corrosion product close to the metal. Oxidation of soluble divalent tin by O₂ at the anodic sites to form insoluble SnO₂ – xH₂O, is suggested to explain the corrosion protection afforded by alloying with tin. Tin was enriched in the corrosion products while no lead was found. The zinc/copper relation was higher in the corrosion products than in the alloy composition.     

Enhanced corrosion resistance of Ni–W alloy with inclusion of TiN nanoparticles by electrodeposition method

Abstract

Electrodeposition of Ni–W–TiN nanocomposite on mild steel substrate from an ammoniacal citrate bath containing dispersed titanium nitride has been demonstrated. The structure, surface morphology, composition and corrosion resistance properties of the nanocomposite deposits have been characterised by using various techniques. X-ray diffraction analysis (XRD) of the electrodeposited Ni–W–TiN nanocomposite shows that it is fcc crystalline. Scanning electron micrography (SEM) reveals smaller grains and uniform distribution of the titanium nitride in the alloy matrix. The microhardness of the nanocomposite coatings is higher than that of the alloy. The corrosion resistance of the electrodeposited nanocomposite evaluated by electrochemical impedance and Tafel polarization studies showed that the Ni–W–TiN nanocomposite is more corrosion resistant than the Ni–W alloy deposit. The finer grain and uniform distribution of the titanium nitride in alloy matrix favour the enhanced microhardness and corrosion resistance of the nanocomposite.     

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.     

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 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.     

Effects of annealing heat treatment on the corrosion resistance of Zn/Mg/Zn multilayer coatings

Zn coatings alloyed with magnesium offer superior corrosion resistance compared to pure Zn or other Zn-based alloy coatings. In this study, Zn/Mg/Zn multilayer coatings with various Mg layer thicknesses were synthesized using an unbalanced magnetron sputtering process and were annealed to form Zn-Mg intermetallic phases. The effects of the annealing heat treatment on the corrosion resistance of the Zn/Mg/Zn multilayer coatings were evaluated using electrochemical measurements. The extensive diffusion of magnesium species into the upper and lower zinc layer from the magnesium layer in the middle of the coating was observed after the heat treatment. This phenomenon caused (a) the porous microstructure to transition into a dense structure and (b) the formation of a MgZn₂ intermetallic phase. The results of the electrochemical measurements demonstrated that the heat treated Zn/Mg/Zn multilayer coatings possessed higher levels of corrosion resistance than the non-heat treated coatings. A Zn/Mg/Zn multilayer coating with MgZn₂ and (Zn) phases showed the best corrosion resistance among the heat treated coatings, which could be attributed to the reduced galvanic corrosion effects due to a small potential gradient between the MgZn₂ and zinc.