6
How the OnGuard Works
Technology
Corrosion on electronic components and contacts and on precious and semiprecious metal artifacts is characterized by
the buildup of various chemical reaction products (films) which form when corrosive gases come into contact with the base
metal. The sources of these gases often result from the types of processes being operated at the industrial site, chemicals
in use and/or general atmospheric contamination. They may also be transported from an adjacent facility so careful
attention should be given to potential sources of atmospheric corrosion outside the facility boundaries as well.
Typical corrosive gases are: hydrogen sulfide (H2S), chlorine (Cl2), oxides of sulfur and nitrogen (SO2, SO3, NO, NO2,
etc.), ozone (O3), ammonia (NH3), and hydrogen fluoride (HF). Therefore, corrosion films may be composed of reaction
products such as copper sulfide and oxide (Cu2S and Cu2O) on copper and silver sulfide and chloride (Ag2S and AgCl) on
silver. These reaction products cause an associated mass gain which can be directly correlated to the average corrosion
film thickness, in angstroms (Å), on the metal surface.
Measuring atmospheric corrosion on metallic surfaces has been done with a variety of techniques. The simplest and most
reliable is the measurement of the mass gain on a metal sample strip (often referred to as a “coupon”). The OnGuard
uses a patented technology to measure the corrosion that will occur on copper and silver surfaces using quartz crystal
microbalance sensors plated with either copper or silver. The plated crystal has a natural resonance frequency based
upon its mass. As corrosion films are formed, the sensor mass increases by the mass of the contaminant gases that have
reacted with the base metal plating. As a result, the crystal’s resonance frequency decreases.
By applying the proper conversion factors contained in the software of the OnGuard, the corrosion buildup that occurs
over the life of the corrosion sensor can be determined. This is referred to as Cumulative Corrosion. The rate of
corrosion buildup over a given time period can also be determined. This is termed Incremental Corrosion.
Cumulative Corrosion
Cumulative Corrosion is the total amount of corrosion that has accumulated on a metal surface. Corrosion is an ongoing
process, occurring when the conditions are correct for additional reaction of the contaminant gases with the base metal.
For surfaces like copper and silver, the onset of corrosion is evidenced by the appearance of a slight tarnish, which is the
actual corrosion film. Progressively, this tarnish gets darker and thicker. Cumulative Corrosion is reported to the
OnGuard in terms of angstroms (Å), which is 10-10 meters.
The OnGuard corrosion sensors have the capability to accumulate 4000 Å of Cumulative Corrosion. At this stage of
Cumulative Corrosion, the base metal copper and silver corrosion sensors will be completely black. If the OnGuard is
being used to monitor the effects gases have on other metallic surfaces, whether they are precious artifacts or electrical
circuit contacts, either visual tarnishing or electrical failures will have occurred prior to the OnGuard’s corrosion sensors
reaching their 4000 Å limit.
Incremental Corrosion
Incremental Corrosion is the amount of corrosion that has accumulated on a metal surface over a specific time period.
Incremental Corrosion can also be referred to as the rate of corrosion buildup. The OnGuard reports Incremental Corrosion
in terms of angstroms per twenty-four hours (Å/24 hours). The Incremental Corrosion is an indication of the severity, or lack
of severity, of a corrosion-causing event. In the OnGuard, the Incremental Corrosion is recalculated every 15 minutes for the
previous 24-hour period. Because Incremental Corrosion is updated every 15 minutes, significant changes in the rate of
corrosion can often be determined within a 15- minute time period. Less significant changes in the rate of corrosion may take
longer to detect.
