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  • Brian Smith

Can lab corrosion testing predict real world performance?

 

Once arrangements are made to corrosion test a customer’s specimens for a given duration, it is not uncommon for the customer to ask, “How does that translate to the real world?” Unfortunately, it doesn’t.  Why?  It’s complicated.


That’s not what a customer wants to hear, and it is the reality of laboratory testing with its controlled variables vs. the very complicated “real world.”  What follows is why it is so difficult to make a correlation.


The most common laboratory corrosion resistance test remains 5% Neutral (pH) Salt Spray, often referred to as “Fog.”  This test has developed over the past 90-some years into two very well-known forms, ASTM Practice B117, and ISO 9227.  These standards describe how to construct and operate a 5% Neutral Salt Spray corrosion exposure test, and what variables are to be controlled or at least recorded.  Those include:  Exposure zone temperature and humidity, the 5% sodium chloride fog/spray pluviation or condensation rate, the pH of the condensed salt electrolyte, and verification of the strength of the sodium chloride in the corrosive electrolyte.

white enclosure with instrumentation and control panel on the side

The 5% Neutral (pH) Salt Spray standards dictate the various test setpoints such as 4-6% sodium chloride mist at 35°C exposure temperature and nearly 100% relative humidity.  There are thousands of these “salt spray” corrosion test chambers in operation in the world, and the only places on the planet where these same conditions exist are in these 5% Neutral (pH) Salt Spray exposure chambers. 


Corrosive “Real world” conditions vary so greatly geographically that naturally-occurring corrosive actors present in Miami, FL are very much different from those in Phoenix, AZ, or in the West Midlands near Manchester, UK.  There are just too many variables to consider, and drawing any sort of meaningful correlation between laboratory corrosion test conditions and those in Sevilla, Spain, for example, requires exposing identically-prepared specimens under both conditions, and examining the samples and comparing them over time.  Using that example, Sevilla is hot and dry; it is not uncommon to regularly exceed 40°C during the summer at the height of the day.  Sevilla has little sodium chloride and little humidity, so the accelerating aspect of the chloride is absent there.  Consequently, it will take considerable time for naturally-exposed samples in Sevilla to exhibit the same degree of corrosion that identically-prepared specimens that were placed in 5% Neutral Salt Fog. 


cars driving down a wintery road in the city
Road deicers in wintery climates put corrosion challenge corrosion resistant coatings.

Sevilla is an extreme contrast, however, with laboratory corrosion testing.  A more comparable natural environment might be Key West, FL, or San Diego, CA, directly on the coast.  Or a northern location where deicers using chloride compounds of sodium, magnesium are applied to roads for four or five months out of the year.  Yet drawing a direct comparison is not possible without undertaking an extensive, time- and money-consuming correlation study. 


Concrete and steel bridge under construction
Corrosion resistant coatings protect valuable coastal infrastructure like bridges.

It is precisely because world climate conditions vary so much with geography and location that results from a simple five- or six-variable laboratory exposure cabinet cannot readily and reliably be compared to identical samples exposed naturally elsewhere.  In fact, Sections 3.2 and 3.3 of ASTM Practice B117-19 5% Neutral Salt Fog cautions the user to avoid drawing conclusions between performance of samples in such a chamber and those exposed naturally without undertaking an involved, proper correlative study. 





  


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