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99285 FAILURE OF ZINC-PLATED SELF-TAPPING SCREWS AND BELLEVILLE WASHERS USED TO INSTALL SPOILER BARS IN CAST IRON DRYERS

Picture for 99285 FAILURE OF ZINC-PLATED SELF-TAPPING
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Product Number: 51300-99285-SG
ISBN: 99285 1999 CP
Author: Robert M. Bruscato, Harvey M. Herro
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During a routine internal inspection of 39 dryers containing spoiler bars, hundreds of broken selftapping screws used to install those bars were found. Many other screws, as well as many Belleville washers, which spring load the bar assembly against the shell, were found to be severely cracked. The spoiler bar system had been in service for 27 months. Fortunately, none of the bars had broken loose inside the dryers. Failures were the result of hydrogen embrittlement which produced the cracking. Hydrogen was produced by a galvanic corrosion cell established between the zinc plating and the hardened washers and hardened self-tapping screws. Minor chemical/pH upset conditions typical in paper mill steam systems may have accelerated the activity of the galvanic cell. This, in turn, enhanced the evolution of hydrogen. Once the zinc was consumed, hydrogen evolution ceased, and the driving force for cracking was eliminated.
During a routine internal inspection of 39 dryers containing spoiler bars, hundreds of broken selftapping screws used to install those bars were found. Many other screws, as well as many Belleville washers, which spring load the bar assembly against the shell, were found to be severely cracked. The spoiler bar system had been in service for 27 months. Fortunately, none of the bars had broken loose inside the dryers. Failures were the result of hydrogen embrittlement which produced the cracking. Hydrogen was produced by a galvanic corrosion cell established between the zinc plating and the hardened washers and hardened self-tapping screws. Minor chemical/pH upset conditions typical in paper mill steam systems may have accelerated the activity of the galvanic cell. This, in turn, enhanced the evolution of hydrogen. Once the zinc was consumed, hydrogen evolution ceased, and the driving force for cracking was eliminated.
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