Abrasive Blasting Tech Tips

BlastTalk: Clearing the confusion on surface chlorides testing

And recommendations from the latest ISO 8502 standards


The measurement of surface chlorides is such an important topic in the abrasive blasting industry as proper surface preparation prior to the application of protective coatings, is paramount in ensuring long-term corrosion protection.

As we know, surface chloride testing relates directly to coating life performance and asset maintenance costs. It is probably unsurprising that over the years, we’ve seen issues in the field where contractors and coating inspectors question erroneous readings from surface chloride tests.

It can result in confusion and sometimes, unnecessary conflict in managing asset owner expectations. We understand that while the standards specified are the same, the interpretation and techniques used can vary greatly.

The key to this is to quantify the ‘actual’ surface chlorides and clear the confusion on elements that ‘do’ impact coating life and other natural elements that ‘do not’ have any impact to contamination. For example, our garnet from Australia has natural occurring bonds with calcites/calcium carbonate, which is also sometimes known in the industry, as a “Corrosion Inhibitor”. Carbonates, Bi-Carbonates and Hydroxides which can be found in natural abrasive minerals, are non-corrosive.

However, the testing gauges we are familiar with, such as the conductivity meter, records a total reading of all elements regardless of their corrosive capacity. Whilst these elements in total, contribute to a conductivity reading, they are non-corrosive elements that are typically in the make-up of natural, almandine abrasive materials.

As they dissolve in solution, these elements, when measured in terms of conductivity, will result in an irrelevant reading.

Moreover, the latest ISO updates released in July this year has recognized that conductivity is an inaccurate measure for determining corrosive contaminants and the importance of blank testing.

The changes then highlight, that we move away from a conductivity test to the Ion Detection Tube analysis utilizing the sleeve extraction methods, which measures chloride ions more specifically.

The updated ISO8502-6 and ISO8502-9 standards recognize that an erroneous reading is possible by the irrelevant contribution of carbonates, bi-carbonates and hydroxides.

Alternative test methods such as ISO 8502-5 and the new ISO 8502-15, which provide accurate instructions to obtain results for ion specific contamination, should now be the used as the primary surface preparation test method to ensure optimum coating protection.

More on the Ion Specific test which uses ion detection tubes. The results from this in-field test method have been found to be more accurate. Compared to the solubilization method, we found that this method provides surface chloride measurements that were not only within specification, but also vastly different from the conductivity or conductometric readings – which includes calcium carbonates, bicarbonates and hydroxides as corrosive elements.

Therefore, the latest ISO standards would have cleared up most of the misconceptions. We see that most specifications have an allowable tolerance of surface chlorides in the upper range that can be easily met through the normal use of the Ion detection tube & sleeve test method.

The update is a recognition that technology has changed since the Bresle method was introduced, with measurement tools now available that provide Ion specific testing to specify an abrasive that provides maximum coating protection.

Key changes on ISO 8502-6:2020 and ISO 8502-9:2020 for users of GMA Garnet™ products

In the 2020 update, ISO has recognized that conductivity is an inaccurate measure for determining only corrosive contaminants and, the importance of blank testing, thus:

  • Specifying multiple extraction methods to perform contaminant analyses. This allows other equipment to perform ion specific testing to achieve true corrosive salt readings.
  • Correcting the Standards to show that the resulting total surface density calculation of water-soluble salts cannot be assumed to be mostly chlorides and sulphates.
  • Recognizing that abrasives, such as garnet, have natural minerals which are non-corrosive that result in a higher conductivity reading. GMA Garnet™ is a good example of this.
  • Mandating the requirement for blank testing to be conducted and be part of the analysis to ensure accurate results. Why? Test patches vary in quality greatly and as a result some patches contributed to false, high conductivity readings.