Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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39 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 A: Usually, Zn anodes have been used on ballast tanks with no problems. If the anodes foul up, they can be cleaned before the next batch of liquid is pumped into the tank, although I don't see why they should foul up. On the other hand, if it's coated with a good-quality coating and inspected periodically, leave it alone. A: Tere has been some thought about thermite sparking between Al alloys (especially higher-Mg types) and rusty steel. Tis has been debated within the coal mining industry where methane gas (CH 4 ) formed may be present. Perhaps the most accurate description is "fric- tional smearing of aluminum alloy onto a steel surface under high specifc pres- sures" ( J.C. Bailey, Trans. of the Institution of Mining Engineering, January 1959). From my cursory glance at this and other literature, the pressure needs to be very high for this reaction to occur. Although the spark can be produced in a laboratory, I believe that there is no proven evidence of it having caused a real-life fre or explo- sion. Steel-on-steel and steel-on-rock can also spark, of course. Cathodic protection survey in snow Q: Is there a recommended practice for conducting a cathodic protection (CP) survey in conditions where there is ice or snow cover on the ground? A: You can do surveys through snow and ice as long as your reference cell makes direct contact with the soil and the soil/ground is not frozen. In some situations the snow acts like a blanket and will keep the ground from freezing. If the ground is frozen, wait until it thaws. A: I have not seen any recommended practice on this issue, but working in icy and snow y conditions never gave diferent results from those on soil. Tere was an added advantage of getting better ground-to-half cell contact, however. I did compare my survey results with previous surveys and found little or no diference. A: If we had to wait for the ground to thaw, we would never get the surveys completed in Canada. One simple method is to use antifreeze solution to wet the ground where you want to take the reading. We use plumbing antifreeze that is nontoxic for this purpose. Most hardware and recreational vehicle outlets stock the antifreeze in Canada. Potential reading and test hole location Q: In our plant complex, we have a huge underground piping network. For the past 15 years, this piping network has been protected by an impressed current cathodic protection (ICCP) system. Our normal monitoring procedure is potential measurement through a number of test holes randomly distributed over the plant. There are no special cables connected to each pipeline for each test hole to measure the potential by a copper/copper sulfate (Cu/CuSO 4 ) reference electrode (CSE). We connect the voltmeter to the exposed grounding network cables that are connected to the steel structures and all underground piping. I have the following questions: 1) Is using the exposed grounding cables efective for the CP potential measurements, and does this method give the actual data? 2) If the test hole is located between two underground pipelines, what potential reading will we get—the actual for each, the average, or something else? 3) What is the extent of the IR drop problem in this case? 4) If I connect one terminal of the voltmeter to one grounding cable, can I measure the potential in several test holes around that grounding cable because I know that all the underground piping is connected to the grounding network? A: Following are my answers to these questions: 1) Yes, because the grounding grid is connected to the underground piping network and belongs to the same protected structures. 2) and 3) Te average of them, but you must be careful because of the distance between where you put the half-cell and where the pipes are located. If the distance is too large, you will measure the pipe potential plus an IR component because the soil is between the half-cell and the pipes. Remember that placing the half-cell closer to the pipe gives the best pipe-to-soil potential. 4) Yes, if all the grounding and the pipes are connected between them. It does not matter where you connect the electrode of the structure; the potential value depends on where you install the half-cell. A: Tis test method is common in power plants. Since the ground- ing cables are connected to the structures that you are protecting, they too are under CP. Connecting the voltmeter to the grounding cable is okay. Te important item is where you place your CSE—as you move it around the area, you will get an average reading of the underground structures. You might also place the refer- ence cell in a remote location to deter- mine the long-line corrosion potentials. Te only cautionary note is that the bare grounding cable will absorb most of your CP current and might also shield some pipes.

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