British scientists have developed a safer and more versatilealternative to chrome electroplating, the coating found on vintage carbumpers, steel camshafts, and fixtures such as door furniture and lightfittings.
Chrome electroplating protects from corrosion and adds anaesthetically pleasing sheen, but chromium comes with serious healthrisks and chromium compounds have been shown to cause cancer.
Speaking at an Institute of Physics conference in Chester, UK,Professor Robert Akid warns that workers are being exposed to chromiumcompounds that are potentially cancer-causing and says that a saferalternative is much needed. He told the conference, Novel Applicationsof Surface Modification, organized by the Applied Physics andTechnology Division of the Institute of Physics, that he and hiscolleagues are developing an alternative that will not only be saferbut cuts down on costs by reducing the numerous processing stagesassociated with conventional electroplating.
Professor Akid, who is Head of the Structural Materials &Integrity Research Centre at Sheffield Hallam University, is developinga so-called "sol-gel technology" which is a colloid with nanoparticlesin a solvent that can form a gel. A metal object is sprayed with ordipped into the sol-gel system and it quickly forms a gel-like layer onthe object's surface. The solvent is then removed by evaporation andthe coating cured, or hardened. Akid says that the sol-gel approach canbe used to coat a wider range of metals than electroplating methods.
Professor Akid said: "These inorganic-organic hybrid coatingshave the potential to become an effective method of producing analternative low-cost anti-corrosion or functional coating. Thetechnology can be formulated and cured to give highly corrosionresistant, ceramic-based coatings. The method uses a range of curetemperatures and coatings are cured rapidly. The chemistry of theformulations has also been developed to provide sol-gel solutions thathave a good shelf life."
There are technical issues yet to be addressed by the Sheffiedteam, however. For instance, a pre-requisite for such anticorrosioncoatings on metals, such as aluminium and other metal surfaces,including zinc, stainless steel, and magnesium, is that they should besufficiently thick, to give adequate lifetime and hard enough to beprotected from scratching and abrasion.
Until now, sol-gel derived layers can be formed only up to twoor three hundred nanometers in thickness with a single-dip. Akidexplains that double dipping to produce multiple coatings is possiblebut this has the potential drawback of a reduction in coatingproperties.
Akid said: "The new type of protective coatings not only havehigh corrosion resistance and don't easily release their constituentions into the environment, but are also non-toxic." Akid and hiscolleagues have used sol-gel mixtures that produce aluminium oxide andsilicon oxide coatings with a chemical component that allows them tobind to the metal component's surface.
The team's preliminary corrosion tests, including so-calledpotentiodynamic polarisation in which an electric current is used toinduce corrosion showed that the coating possesses excellent corrosionresistance properties compared with uncoated samples and otherpre-treatments. The researchers have also carried out mechanical testsand have shown with a simple scratch and bend tests that the coatingsexhibit very good adhesion to the substrate.
As a further test of the durability of the coating, theresearchers also immersed coated components in an exfoliation solutionconsisting of nitric acid and chloride with very high acidity, pH1.This simulates the kind of corrosion that aluminium alloy aerospacecomponents might experience, albeit an accelerated test. Akid explainedthat their test results compared well with similar tests on bare andchromic acid anodised samples. The sol-gel hybrid coating showed littleattack even after almost 200 hours immersion. The chromic acid anodisedcomponents were pitted after this time and so failed the test, whilethe bare simple was extremely corroded. The team used scanning electronmicroscopy (SEM) to identify the nature of the attack, general orlocalised corrosion.
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