| Atmospheric Corrosivity Classification: Two
fundamental approaches have been adopted for atmospheric corrosivity classifications:
- Corrosion measurements in an atmosphere, usually through exposure tests
involving relatively simple coupon devices.
- Measurement of selected atmospheric parameters, notably humidity,
sulfur dioxide concentration and chloride concentration.
Idealized framework for determining atmospheric
corrosivity classifications (schematic)
click on image to enlarge
| One methodology of classification by coupon exposure is
the well-known CLIMAT (Classify Industrial and Marine
Atmospheres) test, reportedly dating back to 1955. 
In these tests, helical aluminum wire is tightly wrapped around a threaded bolt. It was
established that such aluminum wires had the greatest sensitivity in industrial
and marine atmospheres when using a copper and steel bolt respectively.
Because of this enhanced sensitivity, CLIMAT coupons can be conveniently exposed for a
short time period of three months, thereby allowing seasonal variations in corrosivity to
be determined. The enhanced sensitivity also facilitates differentiation of atmospheric
corrosivity on the micro-environmental scale. CLIMAT corrosivity indices are derived from
the percentage mass loss measured on the aluminum wires after the exposure period.
The CLIMAT methodology has recently been selected for monitoring atmospheric corrosion
damage to historical monuments in Kingston, Ontario. Atmospheric corrosion damage to a
cannon from the First World War is illustrated below.
 
Source: Article: "Businessman, professor care for Kingston's corroding
cannons", Kingston this week, August 24, 2001. |
| The ISO 9226 methodology also uses a helical coil of
material as an coupon type exposure test but the standard test period is longer than in
the CLIMAT test. The Instrument Society of America's ISA-71.4 methodology is directed at
indoor corrosive atmospheres affecting
process measurement and control systems. Corrosivity classifications are based on the
thickness of oxide films formed on copper coupons over a thirty day period. Other types of
metal coupons (silver, zinc, lead) have also been utilized for similar corrosivity
monitoring in specific indoor environments.
The ISA methodology has been adapted in a commercial instrument based on highly
sensitive quartz crystal micro balance measurements, facilitating continuous
("on-line") classifications in much shorter time frames. Indoor corrosivity
monitoring with such coupons/devices has found applications in museums (conservation
purposes). |
References/Literature:
ASTM G116 Standard, "Standard Practice for Conducting Wire-on-Bolt Test for
Atmospheric Galvanic Corrosion", American Society for Testing and Materials, West
Conshohocken, PA.
ISO 9226 Standard "Corrosion of metals and alloys - Corrosivity of atmospheres -
Determination of corrosion rate of standard specimens for the evaluation of
corrosivity", International Organization for Standardization (ISO), Geneva,
Switzerland.
Instrument Society of America Standard ISA-71.04 "Environmental Conditions for
Process Measurement and Control Systems: Airborne Contaminants", Instrument Society
of America, Research Triangle Park, NC.
Links:
Indoor atmospheric corrosion monitoring
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