Enhancement of pure titanium localized corrosion resistance by chemical oxidation

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Titanium and its alloys are widely appreciated for their high corrosion resistance in environments that are critical even for high-end stainless steel (duplex UNS S31803 and UNS S32750), such as concentrated chlorides [1]. This resistance is due to a thin (1.5 nm – 10 nm) [2] but compact oxide layer that is naturally formed when the metal is exposed to the air. For this reason, titanium is used where other metals would fail, such as offshore, acid environment, aerospace, automotive, high temperature, chemical & food industry, marine hydrometallurgical application and nuclear fuel wastes containment [3-5]. In such aggressive environments, required corrosion resistance may exceed pure titanium one, therefore surface treatment that rely on the thickening and tuning of natural titanium oxide were developed. Among them anodizing is the easiest and cheapest, and was investigated in past works [5-6]. However, in case of already installed part, localized treatment, small part or difficult geometry, anodic oxidation could be un-feasible. In these cases chemical oxidation would help to increase corrosion resistance of the part. Among the chemicals that induce titanium oxidation, NaOH and H2O2 are the most common and manageable. The aim of this work was to develop a suitable, implementable and optimized treatment to increase commercially pure titanium corrosion resistance up to the level achieved by competing technique such as anodizing. The corrosion resistance achieved by a treatment was classified using a previously developed procedure, that promotes localized oxide breakdown in 0.5 M bromide solution. The two chemicals were used at 10 M concentration, the effect of temperature increase from room T up to 90°C, of treatment duration from 1h to 72h, of solution volume to treated surface ratio, and of agitation condition was studied. Results showed the existence of a plateau after which any increase in treatment duration or temperature doesn’t lead to higher corrosion resistance. This plateau is achieved at shorter time at higher temperatures (e.g. 6h at 60°C and 24h at room T). Corrosion resistance achieved with this plateau is comparable to the one obtained with anodizing at low voltage (20V-40V). Although similar corrosion resistances were obtained using H2O2 instead of NaOH, the time required is reduced by 75%. However, being sodium hydroxide easier to handle and storage, 24h exposure to NaOH 10 M is recommended for in-situ or occasional treatments, while H2O2 10 M for 6h is suggested for industrial application on difficult geometry or small parts. Future development will investigate the possibility to use chemical oxidation treatment to recover damaged anodization to take advantage of both the tunability of anodic oxidation and the portability of chemical treatments.

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Journal: TechConnect Briefs
Volume: 1, Advanced Materials: TechConnect Briefs 2018
Published: May 13, 2018
Pages: 244 - 247
Industry sector: Advanced Materials & Manufacturing
Topics: Coatings, Surfaces & Membranes
ISBN: 978-0-9975117-8-9