Nick Eyre

Corrosion Failure Analysis

For a project for my Failure Analysis & Prevention class at Olin College, I worked in a team of three to explore the severe corrosion of anodized aluminum on an Allied Techcut 5 Precision Sectioning machine. After several years of use, the saw had been taken out of service and replaced due to an extraordinary amount of corrosion on a number of the surfaces of the saw. The saw is primarily constructed of black anodized aluminum, machined and sheet parts. Steel fasteners and a stainless steel coolant reservoir are also present. From specifications, the coolant used in the machine is known to be very slightly basic.

Energy dispersive x-ray spectrometry (EDS) was used to determine the chemical composition of the base aluminum alloy and the corroded specimen. Multiple tests were taken and the results were processed to ignore carbon and the noble gases. Results showed an unusual amount of copper in the corroded region and the presence of small quantities of additional elements. Furthermore, the percentages of aluminum and magnesium were greatly reduced in the corroded region. An x-ray map was generated across a corroded region of the anodized specimen. The map highlighted increases in aluminum, magnesium and silicon in the corroded region and decreases in nickel and sulfur. This makes sense because as the anodize layer is dissolved, the elements below poke through. Notable however is the substantial increase in copper, which is not an alloying element, in the corroded region.

Using scanning electron microscopy (SEM), we compared the anodize layer from the saw sample to a control sample from an anodized bicycle part (AVID BB7 Disk Brake Caliper) in order to determine whether the anodize layer failed due to it’s thickness or other fault. It was observed that the anodize layer on the saw’s parts was approximately twice as thick as that on the bicycle part (20 um vs 10um). Both layers exhibited numerous small cracks, voids and inconsistencies in the layer.

We attributed the saw’s failure to three sources: basic cutting fluid, an insufficiently tough anodize layer, and poor material choice. The cutting fluid mixed with the slightly basic water produced an environment conducive to corrosion of the aluminum, which wouldn’t have been an issue had the anodize layer not been too fragile. Unfortunately, the anodize allowed the aluminum beneath to contact the electrolytic environment through nicks and around fasteners, producing corrosion. In a redesign, a thicker hard anodize layer may have more effectively protected the aluminum. Once started, the corrosion was hurried along by galvanic interactions with steel components and the copper remains of materials cut. Had the fasteners used been coated to mask their cathodic presence or been made of a material with a similar galvanic potential to aluminum, the corrosion would likely have proceeded at a much reduced rate. All in all, no individual flaw was enough to render the saw unusable so quickly but the combination present did so handily.

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