Standard must change to avoid equipment corrosion
By Chris Muller and Grant Crosley
The November 2011 ISA InSights newsletter included a brief article about corrosion-induced failures in electronic products in industrial environments, explaining how ISA71 is addressing the issue. Here is a more in-depth look at how the re-established ISA71 committee is examining ISA-71.04-1985 to address changes in industry.
Since industry began replacing pneumatic and hydraulic controls with computer control systems, the attack from corrosive gaseous contaminants present in many operating environments has challenged the reliability of these electronic and electrical devices.
Since the European Union directive 2002/95/EC has mandated the restriction of the use of hazardous substances (RoHS), research has shown producing printed circuit boards with lead-free material could lead to products that are more susceptible to corrosion than their tin/lead counterparts. The EU's RoHS Directive restricts the use of six substances in electrical and electronic equipment-mercury (Hg), lead (Pb), hexavalent chromium (Cr(VI)), cadmium (Cd), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). In order to comply with the EU legislation, all of these substances must either be removed or be reduced to within maximum permitted concentrations, in any products containing electrical or electronic components that will be sold within the EU. Manufacturers have made significant investments in new processes that will eliminate these substances, especially lead.
Today, even environments previously considered benign present serious problems as a result of RoHS compliance. Reactivity monitoring now needs to provide a more complete environmental assessment than the monitoring techniques described in ISA-71.04. ISA-71.04 is in need of major revisions to incorporate the use of silver reactivity monitoring as a metric and to take advantage of technology advances in real-time corrosion monitoring.
From the examination of the available corrosion film data for copper and silver components, it is difficult to accurately determine the corrosive potential of an environment when following the current methodology of ISA-71.04. Specifically, copper-only reactivity monitoring cannot conclusively determine the presence or absence of environmental chlorine or sulfur oxides. Reporting only copper corrosion, even with a breakdown of the individual films, could result in an incorrect assessment of the environment in question.
ISA-71.04-1985 provides a classification system using corrosion (or reactivity) monitoring to determine the corrosive potential of an environment towards electronic equipment. Reactivity monitoring involves placing specially prepared metal strips into the environment. These corrosion classification coupons would be exposed for a period of time and then analyzed to determine the thickness of the corrosion films that had formed. Reactivity monitoring now needs to provide a more complete environmental assessment than the monitoring techniques described in ISA-71.04. The standard presents manufacturers and users of electronic devices and computer control systems a classification system to gauge the corrosive potential of an environment. It establishes environmental classifications and severity levels within each classification, according to the type of contaminant.
In the 25 years since publication of ISA-71.04, research in environmental classification via corrosion monitoring has revealed this: One of the main limitations in using copper alone for environmental monitoring and assessment is the presence or absence of environmental chlorine, a particularly damaging contaminant to metals, cannot be accurately determined. Silver, on the other hand, is extremely sensitive to chlorine. For this reason, silver coupons, in addition to copper coupons, have been used virtually since the standard was published to provide what is known to be a more accurate assessment of the corrosion potential of a local environment.
ISA-71.04 continues to be a useful tool for characterizing the corrosive potential of an environment. However, many have acknowledged the shortcomings of the standard by using combination copper/silver corrosion classification coupons. New data for silver- and gold-plated coupons established the need to review the standard for the applicability and reliability of copper-only environmental reactivity monitoring. By using copper and silver coupons for this monitoring, it has been shown that the subject environment can be more accurately characterized as to the severity class(es) and type(s) of contaminants present. Any remedial actions recommended as a result of this monitoring would be more concise, and consequently, less expensive. The addition of gold coupons provides even further definition of the subject environment.
The primary consideration in updating the standard is incorporating silver as a quantifiable metric for corrosion risk assessment. The committee hopes to establish a corrosion coupon database, to be set up with data from paired copper and silver coupons exposed in industrial and non-industrial environments, with an indication of whether the equipment in the spaces monitored are RoHS-compliant. Solicitations are being made for contribution to this database, which by some accounts could consist of data from more than 70,000 pairs of coupons.
For more information on ISA71, contact Ellen Fussell Policastro at email@example.com. To read this paper, "ISA Standard 71.04: Changes Required for Protection of Today's Process Control Equipment," in its entirety, visit www.isa.org/link/StandardsINT.
ABOUT THE AUTHORS
Chris Muller is a technical director at Purafil, Inc., in Doraville, Ga., and is co-chair of ISA71 (firstname.lastname@example.org). Grant Crosley is a general manager in reliability and manufacturing systems at Visy Pulp & Paper in Campbellfield VIC Australia (email@example.com).