Mechanical Technology July 2015

⎪ Structural engineering materials, metals and non-metals ⎪

Material engineering in practice: Corrosion is everywhere

work focused on more specific plant related corrosion problems, we are looking at the research of stress cor-

In this issue’s Wits on Materials column, Josias van der Merwe of the School of Chemical and Metallurgical Engineering talks about corrosion and some of the research being undertaken to combat its effects.

rosion cracking as well as metal dusting. Some of these forms of corrosion are not found everywhere but only in very specific industrial processes. When they occur, their presence can be extremely limiting and a huge stumbling block to processes such as those used in refineries and power generation plants. Corrosion clearly affects all of our lives. q

R ecently I asked my students to write about their experiences with corrosion, just in general life where they have come across problems caused by corrosion damage. The students are from a wide range of backgrounds with some growing up in more rural conditions. Some of the stories the students related impressed on me the extent of corrosion and how widely it influences our lives. The stu- dents from the more rural backgrounds related issues with vehicles and support structures that corroded. The consequences varied from being just a hindrance to life threatening. One student described how a modified ox- wagon failed after a small modification did not take corrosion into consideration. Another student described how the family put up a small structure to hold a large water tank and they were quite surprised when the structure failed unexpectedly and almost fell on top of another fam- ily member. After closer examination they found that the structure failed as a result of excessive corrosion of one of the structural members. Another student reported on a water pipeline that burst. This was also in the news because it caused tremendous damage and loss of life. And several students reported on the effect of atmospheric corrosion on, especially, roofing. This fits in well with research that we are involved with at the School of Chemical and Metallurgical Engineering. One of our students is investigating the extent of atmospheric corrosion in the greater Johannesburg Metropolitan. This is a much-needed study since a gener- alised corrosion map for South Africa was set up a number of years ago based on a limited number of sites. Therefore, one of our PhD students (Janse van Rensburg) has decided to focus on studying atmo- spheric corrosion after a number of years performing atmospheric corrosion testing as an Eskom consultant. The investiga- tion considers close to 60 different sites

that are widely spread over the whole Johannesburg Metropolitan Area. The initial results have been very interesting, describing the effect of in- dustrial sites that contribute to most of the increased corrosion rates found, the effect of wind direction. High rainfall and seasonal changes were highlighted. What has been very clear is that atmospheric corrosion in a region cannot and should not be described by a single point mea- surement. These results will be presented as a corrosion map, a guide that can be used to compliment climate condition monitoring. In the South African context it has been very important to consider and implement beneficiation of our mineral resources. Ruthenium is one of the met- als forming part of the Platinum Group Metals (PGM) and is one of the least ex- pensive metals in this group. It has been found that ruthenium provides excellent corrosion resistance to stainless steels (Myburg et al., 1998; Potgieter, 1991; Tomashov & Ustinskii, 1990), but is still too expensive and cannot be feasibly used as a bulk alloying element. For that reason we have been investigating the application of ruthenium rich layers to stainless steel, specifically for sulphuric acid service. The im-

References 1. Myburg G, Varga K, Barnard W, Baradlai P, Tomc- sányi L, Potgieter J, Van Staden M: (1998). Sur- face composition of Ru containing duplex stainless steel after passivation in non-oxidizing media. Applied Surface Science, 136 (1-2), 29–35. doi: 10.1016/S0169-4332(98)00326-2. 2. Potgieter JH: (1991). Alloys cathodically modi- fied with noble metals. Journal of Applied Elec- trochemistry, 21(6), 471–482. doi:10.1007/ BF01018598. 3. Tomashov ND & Ustinskii EN. (1990). Cor- rosion behaviour of chromium-ruthenium alloys in sulphuric acid solutions at active dissolution potentials. Protection of Metals (English Translation of Zaschita Metallov), 26(1), 99–102. Retrieved from http://www. scopus.com/inward/record.url?eid=2-s2.0- 0025480610&partnerID=tZOtx3y1.

provement found in corrosion resistance is considerable, with only small ruthe- nium additions. This is extremely beneficial as it makes the use of ruthenium a viable option for sulphuric acid service. The corrosion re- sistant layers are ap- plied by laser alloy- ing and cladding and have also been used for both general cor- rosion to replace more expensive materials such as Hastelloys. In similar

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