Photo-Corrosion of Different Metals during Long-term Exposure

T. David Burleigh*, Christopher Ruhe**, and Jarred Forsyth

*Materials & Metallurgical Engineering Dept, New Mexico Tech, Socorro, NM 87801

**National Steel Corporation, Portage, IN 46368

ABSTRACT

The effect of ultraviolet light on the corrosion rates of different metals was studied in two separate investigations. Metal samples were immersed in a flowing fresh water discharge stream for either three or five months under both ultraviolet light (UV) and dark conditions. Weight loss results demonstrated that pure zinc, carbon steel, aluminum 6061, pure copper and pure silver all experienced photo-corrosion, that is, a greater weight loss under exposure to UV light versus in the dark. The photo-corrosion is explained with models based on the photo-voltages measured on corroded metal samples.

INTRODUCTION

It has been reported in the literature that the passive oxide films on metals are semiconductors and these passive films exhibit photo-effects such as photo-currents and photo-voltages in the presence of light [1-3] . Some researchers have reported photo-inhibition and photo-corrosion for different metals exposed to ultraviolet light under laboratory conditions [4-11]. Lenhart et al reported that white light inhibited the pitting of nickel in a buffered chloride solution [4]. Schmucki and Bohni reported on the UV photo-inhibition of corrosion of iron immersed in a borate buffer solution [5]. MacDonald et al reported on the photo-inhibition of pitting corrosion of 304 stainless steel in chloride solutions [6-7] . Kalinauska et al reported on the photo-corrosion of zinc in chloride solutions [11] .

The question is, are these photo-effects simply interesting laboratory artifacts, or is there a significant change in the corrosion rate of the metal? This paper describes the results of two separate field tests of photo-corrosion of metals in flowing fresh water.

EXPERIMENTAL PROCEDURE

The results herein are from two separate investigations, the first by Ruhe and Burleigh in 1997-1998, with nine metals tested (Table 1) and second investigation by Forsyth and Burleigh in 1999-2000, with ten metals tested (Table 2). In each investigation, the metal samples were immersed in the discharge stream of the Franklin Regional Municipal Sanitary Authority (FTMSA) where ultraviolet light (Trojan System UV3000 [13] ) was used to disinfect the treated water before it was discharged into Turtle Creek. The Trojan UV3000 system consisted of one hundred and twelve parallel, 27 Watt, 58 inch length, fluorescent UV lamps. One set of duplicate metal samples was placed in the discharge stream in the midst of the Trojan UV3000 lamps, while the second set of duplicate metal samples was placed upstream and experienced only the normal day-night cycles ("dark"). The dark Ruhe samples were about 2 m below the water level and the other samples were immersed in 0.3-0.5 m of water. Once every week or two, the disinfection channel was drained for one to two hours for rinsing.

Complete details on the exposure conditions will be available in print. [14] This detailed report will also include the results of laboratory exposures that accompanied the field tests described in this report.

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