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13 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 THE BLOG Continued f rom page 11 primer has come into contact with SS piping during painting of adjacent carbon steel piping? A: As temperatures approach the melting point of zinc, you will get liquid metal embrittlement of austenitic SS. It is common practice not to apply zinc directly to the SS in the event of a fi re or if someone welds on the material in the future without knowing the zinc is there. A: e paint can be removed by grit blasting. We even use soda blast- ing to remove paint in order to dye pene- trant check for stress corrosion cracking. However, before spending the money or ef- fort to do that, you have to ask the ques- tion, "What are the consequences of fail- ure?" In the unlikely event that there is a fi re and the spot gets hot enough to melt the zinc, or someone welds on the line without grit-blasting it clean, what will happen if it leaks? Evaluating the combi- nation of " likelihood of failure" and "con- sequences of failure" is at the heart of risk-based inspection programs. A: If the SS is a pressure vessel, then take action; if not, you can proba- bly just leave it alone, especially if it is a non- or low-pressure pipe (or even a cable junction box). Coating Tank Bottoms Q: For an on-grade aboveground tank on a sand foundation, with high-density polyethylene second layer containment and cathodic protection (CP) of the bottom plate, should the underside of the tank plate be coated? We have a client who says coating the under- side of bottom plates causes some problems with CP and recommends no coating. Apparently when welding the base plates together, the coating is overheated locally and will then disbond. The CP will not protect the disbonded areas, which are in effect a shield. Would you recommend leaving an uncoated strip at the plate edges to avoid the disbonded zone, and if so, how wide? A: We generally specif y 300 µm of coal tar epoxy on the floor plates before laying and welding. For the coated area, use a realistic coating breakdown fi gure and design the CP accordingly. If you have secondary containment, install a perforated nonmetallic tube under the tank close to the tank floor with a pull wire so you can check polarization of the cathode surface. A: If you have a choice here, CP al- ways works best (less amperage and further throwing power) with a coat- ing in place. A: If there is secondary contain- ment, as there is in most cases these days, anodes around the tank do not apply (one needs also to consider the tank diameter and potential shift at the center if peripheral anodes are used). One must use a close anode of the grid or dis- tributed ty pe; under these conditions, coating is the best way to go. We have suc- cessfully done this on more than 200 tanks. e space between the tank and the secondary containment is never >500 mm so anode spacing vs. current distri- bution needs to be carefully considered if the plates are to remain bare.