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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Page 22 of 92

20 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIAL MATTERS This nitrogen generating system, which protects multiple fre protection systems in one building, consists of the nitrogen generator (left), the nitrogen gas receiver tank (center), and a control panel (right) that monitors the nitrogen purity concentration within the multiple pre-action systems. Photo courtesy of South-Tek Systems. Continued f rom page 19 Van Der Schijff and Bodemann comment that the use of galvanized steel is based on the principle of cathodic protection (CP) for corrosion protection—the zinc coating on the interior of the pipe will corrode sacrif icially and protect the underlying steel. They add, however, that under certain conditions, galvanizing the interior surfaces of the pipes has been found to be ineffective for mitigating cor- rosion in sprink ler systems. The low points in sprink ler piping are often wet as a result of the accumulation of water and condensate. Due to the stag- nant nature of sprink ler systems, corro- sion products deposited on corroding metal remain where they are formed. In galvanized pipe, the eff iciency of the gal- vanic CP process decreases as the surface becomes covered with nonconductive zinc oxide. Eventually, the zinc coating is penetrated and the underlying steel expe- riences localized corrosion in the form of pits covered by a mound of oxide called a tubercle. Numerous owners have reported premature galvanized pipe failures in as little as three years after commissioning due to multiple pinhole leaks at locations where penetration of the zinc coating occurred under tubercles. Van Der Schijff and Bodemann report that subsequent investigations found common conditions in all instances of galvanized pipe failure, including the lack of an adequate method for completely draining the f ire protection system after initial hydrotesting and f low testing, multiple pockets of trapped water, local- ized tubercles with underlying pits that penetrated the steel at breaches in the zinc coating, and intact zinc coating that covered sur- faces surrounding the localized tuber- cles. They also note that corrosion of black steel pipe without a protective interior coating ini- tially results in even, uniform thinning of the steel pipe wall with a considerably slower rate of pene- tration. It is only later when signif i- cant amounts of corrosion product deposits have accu- mulated within the piping that the cor- rosion mechanism transitions into localized pitting. In general, once corro- sion has initiated, the time-to-failure for black steel piping is longer than it is for galvanized steel. When compressed air is used as the supervisory gas for dry and pre-action f ire sprink ler systems, oxygen (which makes up ~21% of the compressed air stream) is constantly introduced into the sprink ler system. The oxygen content, in combination with residual moisture in the air stream and/or residual water in the piping, creates optimum conditions for corrosion to initiate. Oxygen is the key driver of electrochemical corrosion in sprink ler piping, Bodemann explains. The nitrogen generator produces a con- tinuous supply of supervisory gas that is comprised of >98% nitrogen, a percentage that has been shown to reduce the oxygen content within the sprink ler system to a concentration where the onset of corro- sion is effectively inhibited. In the long-term exposure test experi- ment, Schedule 10 black and galvanized steel sections were half f illed with water and subjected to a supervisory gas com- prised of either compressed air, 95% nitrogen, or 98% nitrogen. Recent results of pipe samples pulled out of the test envi- ronments after 1,116 days of continuous exposure showed that the corrosion pen- etration rate in black steel pipe was sig- nif icantly slower when exposed to super- visory gas comprised of 98% nitrogen— a 66% reduction of the corrosion rate recorded when the pipe was exposed to compressed air. For galvanized pipe, the 98% nitrogen supervisory gas reduced the corrosion penetration rate by ~94% and prevented the formation of localized pits. Ongoing research, they say, continues to prove that 98% nitrogen supervision is a very effective means of addressing dry and pre-action sprink ler pipe corrosion. To effectively inhibit corrosion, high- purity nitrogen must be continuously dis- tributed throughout the f ire protection system's entire piping network. Banks of high-pressure nitrogen gas cylinders are sometimes used as a source of supervi- sory nitrogen gas. However, cylinders contain a f inite supply of nitrogen, which could be an issue in dry and pre-action f ire protection systems. Because there 20 NOVEMBER 2014 MATERIALS PERFORMANCE

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