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DO <br />Roof <br />SP0178-2007 <br />Weld Inside = Eliminate Crevice Inaccessible for Grinding or <br />Proper Lining Application <br />Inside of Tank or <br />Vessel <br />Shell <br />DON'T <br />Inside of Tank or <br />Vessel <br />FIGURE A7 <br />Roof -to -Shell Joint. Eliminate crevice and lap weld at roof -to -shell joint in a tank or nonpressure vessel. <br />....—_ Pit (Anode) <br />Steel <br />Protective <br />Lining <br />Alloy (Cathode) <br />DO <br />.1— Pit (Anode) <br />Steel <br />Protective <br />Lining <br />Alloy (Cathode) <br />DON'T <br />FIGURE A8 <br />Dissimilar metal (galvanic) corrosion occurs when, for example, an alloy is used to replace the steel bottom of a tank, or, in a <br />similar circumstance, when alloy appurtenances must be a part of the construction of a vessel. If a lining is then applied to the <br />steel and part of the alloy (usually 150 to 610 mm [5.9 to 24 in.]), any discontinuity in the lining exposes a small anode surface. <br />Once corrosion starts, it progresses rapidly because of the large exposed alloy cathodic area to the much smaller anodic area. <br />Without the lining, galvanic corrosion causes the steel to corrode at the weld area, but at a much slower rate. The recommended <br />practice is to apply lining to all of the alloy as well as the steel, thereby eliminating the possible occurrence of a large -cathode -to <br />small -anode surface. <br />NACE International 9 <br />