Properties

Properties of Galvano-Aluminum Layers

Material Properties

Primary material characteristics include the chemical composition and the crystalline structures as well as their orientation. These, in turn, determine the technical and functional properties of the ultimate product.

A scanning electron microscopic image of aluminum plating on steel demonstrates that the galvano-aluminum coating fails to exhibit the columnar structure attained by other processes, which explains why there is no porosity.

Technical Properties

Layer Thickness

Galvano-aluminum coatings have a thickness that is practically infinitely variable. Starting at 7 µm, nonporous films are obtained. To ensure adequate corrosion protection, the layer should preferably exhibit a thickness of 10 - 15 µm. In the presence of a sufficiently thick layer, the method of anodic oxidation, an established technique for massive aluminum materials, may also be used on galvano-aluminum.

Volumetric Weight

Galvano-aluminum has a very low density compared to commonly used electroplating metals. Thus, considerable weight reductions may be achieved, which is a particularly decisive factor in light-weight construction, for instance in the aviation industry. Galvano-aluminum layers, at a density of 2.65 g/cm3, practically equal the common value for highest-grade aluminum of 2.702 g/cm3, which points to the excellent purity and homogeneity of the layers.

Roughness

The roughness of galvano-aluminum layers is largely independent of the current density, which illustrates the relatively strong inhibition of the deposition reaction. By analogy to aqueous electroplating systems, the roughness increases with the layer thickness, ranging from 0.3 to 0.8 µm at thicknesses of up to 20 µm.

Hardness

The hardness of the pure (30 µm thick) galvano-aluminum layer, at a load of 0.015 kg, equals 19 to 25 HV (Vickers hardness). This corresponds to the hardness of unalloyed, rolled, and heat-treated aluminum, for instance of the type AA 1100-0. Layers anodized by a sulfuric acid process, at 0.050 kg, on the other hand, reveal hardness values of 500 - 750 HV as compared to aluminite layers on pure commercially available aluminum with 300 - 400 HV at 0.045 kg.

Ductility

Galvano-aluminum coatings are characterized by their extremely high ductility. Under any flexural test conditions according to ASTM B-489 involving 1 mm thick sheet metal and more, up to a bending diameter of 0.1 mm, the surface - irrespective of the layer thickness - remained absolutely crack-free, which corresponds to a calculated extension of more than 80 %.

Image 1. Coiled copper wires, d = 0.4 mm, Al layer thickness 20 µm

Stress

The internal stress was determined by an x-ray procedure. At layer thicknesses of 10 - 20 µm and 40 µm, respectively, extremely low values of 14 - 18 N/mm2 respectively 9 N/mm2 were obtained.

Adhesion

The adhesion of galvano-aluminum on steel exceeds the shearing force of the aluminum layer itself (95 - 100 N/mm2). Even thermal shock treatment (30 min at 250 °C, immediately followed by immersion in cold water) failed to separate the aluminized layer.

Functional Properties

The functional properties are a primary concern in practical engineering. They are based on the above-mentioned material and its technical properties.

Influence on the Stability of the Substrate

Aqueous metal plating processes usually result in hydrogen charging of the base material, thus causing embrittlement of high-tensile steels. Aluminum deposition completely eliminates this phenomenon, since there is no water (a hydrogen supplier) in an electrolyte bath. Aqueous pretreatment may ultimately cause hydrogen cracking. Steels with a tensile strength of up to 1500 N/mm2, however, may be metal-plated without showing any evidence of hydrogen cracking.

Abrasion

The sulfuric acid (GS) and the alkaline anodizing processes yield highly abrasion resistant anodized layers on galvano-aluminum.

Decorative Properties

Requirements regarding the decorative qualities - obtained by a modern design - and other functional properties of engineering surfaces are steadily increasing. In this respect, galvano-aluminum stands out due to its unsurpassed properties. High-purity aluminum has a dull silvery surface, which remains unchanged during use. Competitive coating metals, such as cadmium, zinc, copper, and nickel, on the other hand, do tend to change their surface qualities very soon.

Chromium-plating of aluminum will also give rise to dull silvery, but also golden-yellow to brown types. Clear or pigmented anodized layers may be deposited on aluminum and its alloys.

Image 2. Pigmented anodized metal pins

Corrosive Properties

As compared to conventional chromatized coatings, electroplated, chromatized aluminum reveals excellent self-corrosion and cathodic protective properties. Under conditions simulating close contact with salt water (off-shore) and wet roads covered with salt, respectively in a 500-hour salt-spray test, aluminum and cadmium remain entirely intact, while the corrosion products of zinc cause a typical white rust. Damaged layers of aluminum and cadmium, too, achieve a comparative degree of protection regarding the steel substrate. Only zinc is unable to prevent red rust formation on the base material.

(Source: TNO-Metaalinstituut in Apeldoorn in the Netherlands)

Image 3. An aluminium plated housing and an untreated housing made of cast aluminum, after 480 hours exposure to a salt-spray test according to DIN 50021