 |
|
 |
 |
Codes and Standards |
|
VIII. CATHODIC PROTECTION USED IN CONJUNCTION WITH PAINTS Cathodic protection can be achieved using either galvanic anodes {zinc, aluminum or magnesium) or impressed current systems as the source of the protective current, As in the case of cathodic protection from zinc incorporated in a paint, the effect of the impressed current is to eliminate or change the direction of the potential difference in the numerator of equation 3. Cathodic protection simply substitutes electrons from an external source for the electrons otherwise generated in a corrosion cell to accommodate reduction of hydrogen ions and oxygen at the cathodic surfaces. The electrical resistance of the coating (RCt) plays an important role in cathodic protection by increasing the "throwing power" of the usually relatively small anodes by enabling the protective current to extend for greater distances from the current source. It has been found that under severe service conditions a combination of a good paint system and cathodic protection is better than either one alone. In addition to the "throwing power" effect, a paint system reduces the current required for cathodic protection by as much as 100 to 1, depending on the condition of the paint. Even when there may be no opportunity for renewal of a paint system, its use can be justified in conjunction with cathodic protection in sea water. This is based on the probability that, in the course of time, the calcareous deposits created by cathodic reactions wilt replace the original paint system in achieving distribution of current and maintaining the level of current required for protection.10 Paint systems used with cathodic protection not only must tolerate attack by cathodic alkali, but must be protected from the danger of blistering by hydrogen which can result from too high a cathodic current density. Cathodic protection is usually monitored and controlled by measurement of the potential of the protected metal. This potential is measured relative to that of an appropriate "bench mark'" reference electrode. One such electrode is a saturated calomel half cell. It is assumed that protection of steel has been achieved when its normal potential in sea water of about -600 mV has been raised to -850 mV. Potential measurements can be used, as well, to avoid hydrogen blistering of paints by restricting the potential resulting from cathodic protection. A conservative maximum polarized potential would be about - 1000 mV versus a saturated calomel half cell. The advantage of a substantially intact paint film having high electrical resistance in connection with cathodic protection is reversed in situations, usually rare, where painted steel immersed in an electrolyte becomes involved in the passage of a "stray" electrical current. Under such circumstances the current is forced to leave the metal at discontinuities in the coating with consequent severe localized attack. This has been observed, for example, on painted ship hulls when an on-shore source of current for electrical welding on a floating ship has been provided with inadequate negative return cables. This leads to a substantial amount of current returning to ground through the water path in parallel with the return cable path. The effect is to increase greatly the anodic potential AP in equation 3 leading to a high corrosion current I concentrated at discontinuities in the coating. X. EFFECTS OF COMPOSITION OF STEEL Self-limiting forms of rust can offer protection to steel under certain conditions of atmospheric exposure. The protective qualities of such rust films are affected by alloying elements and other minor constituents of steel. Copper, chromium, nickel and phosphorus have beneficial effects. Sulfur has the greatest detrimental effect, which can be compensated for by the presence of copper in an amount greater than the sulfur content. Combinations of favorable alloying elements are more effective than the same content of a single beneficial element. This is the case with the so-called "high-strength, low-alloy" steels. As measured by weight loss after exposure in certain corrosive atmospheres for 10 years, these steels showed an advantage over ordinary steel in a ratio of about 4 to t or greater. The advantage of the Iow-alloy steels is even greater when the steels are painted? as illustrated by Figure 4. Painted specimens of a steel of very low copper content have poor resistance to a marine atmosphere as compared with a better steel containing about 0.20% copper and an even better steel containing copper, nickel, chromium and phosphorus. The alloy steel suffered much less spreading of corrosion adjacent to the scribe marks in the paint. Further improvement was achieved by a phosphating pre-treatment of the steel before painting. As measured by weight loss of scribed panels the advantage of the alloy steel over the poorest steel was in the ratio of 10 to 1. The combination of the phosphate pre-treatment and alloying resulted in an improvement to a ratio of 20 to 1. The advantage of a low-alloy steel observed in atmospheric exposure is not duplicated under conditions of immersion. The better performance of the alloy steel in atmospheric exposure is based on the superior protective qualities of the thin film of rust that forms on the alloy steels, while the voluminous hydrated rusts that form on steels under conditions of immersion do not exhibit a similar difference in protective ability. Furthermore, the principal factors that influence corrosion under water, such as dissolved oxygen, effects of organisms and water velocity, are external to the steel rather than related to its composition. Knowledge of the reactions involved in the corrosion of steel combined with a knowledge of how a paint system can impede these reactions and the qualities of a paint system needed to achieve the desired results, as described in the following chapter, along with proper preparation of steel of desirable composition, can serve as an effective guide for using protective coatings to prevent corrosion. |
Copyright © Preferred Tank & Tower Inc., 2008