Iron-zinc alloy layer

Iron can be beneficial in coatings. The iron-zinc alloys formed in hot dipping or sherardizing can be up to 30% more resistant in mildly acidic conditions, but some workers report lower corrosion resistance with some galvannealed coatings. The iron-zinc alloy layers, while continuing to protect... 

In an industrial atmosphere, the best results were obtained with electroplated iron-zinc alloy layers with more than 20% iron (Salt et al., 1965) with a corrosion resistance 30% higher than zinc. Elsewhere, zinc-iron alloy galvanized coatings were as good as coatings with an outer zinc layer. Sherardized coatings were superior to electroplated and equal to galvanized for the same thickness. However, the structure of the alloy layer affects the corrosion resistance, as does its composition. 

Horstmann, D. (1985). Formation and growth of iron-zinc alloy layers. I4th Int. Galv. Conf., Munich, ZDA, London, pp. 6/1-5. 

The adhesion is good compared with electroplated coatings. Thickness can be varied as desired from 5 pm to more than 70 pm. However, the coating is not alloyed with the steel, nor does it have the hard, abrasion resistance iron-zinc alloy layers of galvanized or sherardized coatings. Conversion coatings can be applied. 

In contrast to zinc coatings, massive zinc parts in seawater also suffer from pitting corrosion. In zinc coatings, local corrosion is prevented by the circumstance that the outer layer of pure zinc has a less noble potential than the iron-zinc alloy layer, resulting in cathodic protection. 

In the hot-dip galvanising process (hot-galvanising), layer thicknesses of 0.040-0.150 mm are produced on the steel surface. This coating consists of an iron-zinc alloy layer at the phase limit to the steel and a layer of pure zinc over that. The protective effect of the coating is determined by the layer thickness of both layers. In the initial stress load phase, covering layers are formed by the reaction of the zinc coating with the seawater. After this, a stationary phase of corrosion is reached with mainly linear corrosion rates at an order of magnitude of 0.010-0.012 mm/a (0.39-0.47 mpy). 

Aluminum and chromium are both passive metals in the true sense. Zinc exhibits good corrosion resistance, which is attributed to the formation of an adherent protective layer of corrosion products (this may also be considered as a form of passivity). Chromium, at concentrations >12%, confers passivity to its alloys with iron, and these alloys are cathodic to steel. Under most conditions, both aluminum and zinc are anodic to steel, as are iron-zinc alloys. Iron-aluminum alloys, however, are cathodic and in this respect are similar to iron-chromium alloys. The corrosion resistance of steel is increased by alloying with aluminum or chromium, such increase being marked with the latter. The addition of zinc decreases the corrosion resistance of steel, although, in many cases, a protective layer of corrosion products leads to an apparent decrease in the corrosion rate. 

The effectiveness of cathodic protection probably depends on the composition of the outer portion of the zinc layer. Pure zinc is more anodic than iron-zinc alloys, so bare steel can more readily be protected by zinc than by the alloy. The degree of cathodic protection depends largely on the exposure conditions. 

Plating of a thin iron-phosphorous (Fe-P) layer (2 g/m ) or iron-zinc (3 g/m ) layer on the surface of iron-zinc hot dip or electroplated coatings is also used for automotive applications. The thin electroplated layer is designed to improve formability and to reduce electrocoat cratering tendencies of the iron-zinc alloy coating. 

Zinc is aggressively corrosive to the cast iron kettle and the top section in contact with the zinc alloy layer is lined with chrome magnesite brick. However, lower sections serve as heat transfer surfaces in contact with bullion and corrosion is significant, especially with the two upper castings. Consequently the life of the kettles is only of the order of two years. This imposes a significant cost penalty and adverse economics have caused the Port Pirie operation to replace this elegant approach with the more conventional batch system, using standard kettles. 

Zinc plating layers transformed by a heat treatment at 903 K (630 °C) completely into iron-zinc alloy phase (galvannealing) showed much better behaviour in the exposure zones. The corrosion protective effect of the hot-dip galvanised layers in seawater is thus not determined only by layer thickness, but also by the layer structure [216]. 

In thermal-sprayed zinc coatings, layer thicknesses of 0.050-0.150 mm are the maximum reached. These layers consist mainly of pure zinc, with a certain amount of zinc oxide and pores depending on the layer thickness and spraying technique. The much lower protective effect of spray galvanisation at the same layer thickness is probably due to the lack of the iron-zinc alloy phase. 

The conditions which affect the type of reaction are bath temperature and the composition of iron or steel which is being coated. At 480-520°C the reaction between iron and zinc can be linear with time so that the thickness of the alloy layers will increase in direct proportion to the immersion time and the reaction will continue to be relatively rapid. With some steels (e.g. some silicon-killed steels), the reaction can be linear at the normal galvanising temperature of about 450 C. 

This process, also termed rapid spinning cup (RSC) process, was invented in the early 1980 s contemporarily by Osaka University in Japan[191] and Battelle s Columbus Division in the US)192 Unlike water atomization where water streams or droplets are used to disintegrate a molten metal, a coherent fast-moving liquid layer is used in the RSC process. Liquid quenchants include water, oil, glycerine, and other commercial quenching liquids. The materials atomized with the spinning cup method include a wide variety of metals and alloys such as tin, lead, aluminum alloys, copper alloys, iron alloys (stainless steels and high speed tool steels), zinc alloys and superalloys.[192]... 

All the other metallic alloys (iron, zinc, lead, etc.) that do not form protective and/or aesthetic patina, are usually covered with paint as protection layer. Anyway, many world treasures have been moved indoors after restoration, as in the case of Marco Aurelio and the Venice Horses. 

A hydroxide suppression model first proposed by Dahms and Croll (2) explains anomalous codeposition behavior of zinc-iron group alloys. This explanation was later supported by a number of workers (3) who measured a rise in pH near the cathode surface during the deposition of Zn-Co alloy. In this model it was assumed that the Zn(OH)2 was formed during deposition as a consequence of hydrogen evolution, thus raising pH in the vicinity of the cathode. Zinc would deposit via the Zn(OH)2 layer, while cobalt deposition took place by discharge of Co2+ ions... 

Zinc and its alloys are widely used because they have low melting points and can be easily cast. Therefore, various objects are made from zinc and its alloys. Zinc is widely applied to iron and steel as a protective coating by the process known as galvanization that consists of coating an iron object with a thin layer of zinc. Relative to iron, zinc is an anode, so it is preferentially oxidized. If the coating is broken, the zinc continues to corrode rather than the iron object. When iron is coated with a less reactive metal such as tin, a break in the coating causes the more easily oxidized iron to be corroded at an accelerated rate. 

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