Surface Salt Testing: Importance, Methods and Coating Failure Prevention

Surface preparation is one of the most important steps before applying a protective coating. A steel surface may look clean after abrasive blasting or mechanical cleaning, but it can still contain invisible contaminants. Soluble salts such as chlorides and sulphates are common examples.

These salts can create serious problems when they remain on the steel surface before painting. They can attract moisture from the surrounding environment and form an electrolyte under the coating. Over time, this can lead to corrosion, blistering, osmotic failure, poor coating adhesion, and premature coating deterioration.

This is why surface salt testing is an important inspection activity in industries where steel equipment needs long-term corrosion protection. Testing helps identify soluble salt contamination before coating application and provides an opportunity to clean the surface properly.

In chemical process plants, this testing can be especially important for storage tanks, pipelines, pressure vessels, structural steel, heat exchangers, and other equipment exposed to aggressive environments. Proper testing, surface preparation, and coating inspection can help reduce coating failures and support better asset reliability.

What Is Surface Salt Testing?

Surface salt testing is used to find and measure soluble salts on a steel surface that has already been prepared.

Steel can pick up salt contamination at many points. This can happen during fabrication, shipping, storage, sand blasting, or day to day use. Chlorides and sulphates may come from the air, process chemicals, water, nearby materials, handling, or other exposure in the environment.

The issue is that the salts may not show up to the eye. After blasting, a steel piece can look neat and match the expected appearance, yet still carry soluble salts.

With surface salt testing, you get clear numbers for how much contamination is present. Those values can be checked against the stated project limits or coating rules before any painting starts.

Taking this quick check helps avoid costly setbacks later on. 

Why Soluble Salts Cause Coating Problems

Salt on its own is not the real villain. The trouble starts when it meets moisture. Most coatings are not completely waterproof. They slowly let water vapour pass through over time. When that vapour reaches a salt deposit trapped under the paint film, the salt dissolves and forms a concentrated salty liquid.

This liquid has two harmful effects:

• Osmotic blistering. The salty solution pulls in more water through the coating, like a sponge with a one-way door. Pressure builds up, and the coating lifts into blisters.

• Under-film corrosion. The dissolved salt turns into an electrolyte, which is exactly what steel needs to start rusting. Corrosion then spreads quietly beneath the coating, weakening adhesion and eating into the metal.

Once this cycle begins, it feeds itself. Chlorides in particular are not consumed during corrosion, so they keep working long after the first damage appears. This is one of the most common reasons behind early corrosion failure in coated equipment.

Where the Risk Is Highest in Chemical Plants

Chemical process plants are especially exposed to salt contamination. Some of the equipment that needs extra attention includes:

• Storage tanks, especially those near the coast or handling chemicals

• Pipelines and pipe racks

• Pressure vessels and heat exchangers

• Structural steel and supports

• Equipment that has been idle or stored outdoors for long periods

Harsh operating conditions, chemical fumes, humidity, and temperature swings all speed up damage when contamination is present. Repairing coatings on this kind of equipment is expensive, and the work often requires shutdowns. Catching the problem before painting is far cheaper than fixing it afterward.

Common Methods of Surface Salt Testing

There are several accepted ways to test for soluble salts. The right choice depends on the project size, the required accuracy, and the specification you are following.

1. Bresle Patch Method

This is one of the most widely used field methods. A small adhesive patch with a soft membrane is stuck to the steel. Clean, deionised water is injected inside the patch and moved around to dissolve salts from the surface. The liquid is then drawn out and tested. The method is described in ISO 8502-6 for extraction and ISO 8502-9 for conductivity measurement.

2. Conductivity Measurement

After extraction, the water sample is tested with a conductivity meter. The more salt in the water, the higher the reading. This gives a quick overall picture of total soluble salts. It does not tell you which specific salt is present, but it is fast and practical for routine checks.

3. Chloride-Specific Test Kits

When chlorides are the main concern, specific tools such as titration strips or ion detection tubes can measure chloride content directly. This is helpful because chlorides are usually the most damaging salt for steel.

4. Swab and Wash Methods

Some kits use a swab or a rinse to collect salts from a larger area. These can be useful for rough or irregular surfaces where patches do not stick well. Results are generally treated as a good indication rather than a laboratory-grade figure.

5. Laboratory Analysis

For critical assets or disputed results, samples can be sent to a laboratory for detailed ion analysis. This is also common in failure investigation work, where the exact type and source of contamination need to be confirmed.

What Are the Acceptable Limits?

There is not one fixed number for every case. The salt level that is acceptable depends on what the coating maker says, what the project calls for, and what the parts will face after coating. If the equipment will sit in liquid or be hit by strong chemicals, the limit is usually much lower than it is for mild outdoor air exposure. Some job specs set chloride at just a few micrograms per square centimetre. Other high priority jobs go lower than that.

A good approach is not complicated. Read the coating supplier’s notes first. Also read the project specification before you start. Then write down every test result as you go. 

Tips for Better Testing and Prevention

Good testing is only half the story. These habits make a real difference on site:

• Test after surface preparation, not before. Salts can be hidden or revealed depending on the cleaning method.

• Test at multiple locations. Check welds, corners, pits, and low areas where salts tend to collect.

• Use high-pressure fresh water washing or wet abrasive blasting when salt levels are too high. Dry blasting alone often cannot remove salts from deep pits.

• Re-test after cleaning to confirm the surface now meets the limit.

• Protect the surface after cleaning. Do not leave prepared steel exposed to humidity or sea air. Flash rust and fresh contamination can appear quickly.

• Check your abrasive and water. Contaminated blasting media or dirty rinse water can add salt instead of removing it.

• Keep good records. Documented results help with quality audits, warranty claims, and future maintenance planning.

The Role of Skilled People and Proper Training

Salt testing is simple in concept, but reading the results correctly takes experience. A trained coating expert knows how to choose the right method, avoid contaminated samples, and judge whether the numbers make sense for the service conditions. A seasoned corrosion expert can go a step further by linking the results to the operating environment and the likely damage mechanisms.

This is why corrosion training for inspectors, supervisors, and maintenance teams is so valuable. When the people on site understand why a small number on a meter matters, they take testing seriously and coating quality improves across the board.

How Salt Testing Supports Asset Integrity Management

Coatings are the first line of defence for steel equipment. If that layer fails early, everything behind it is at risk. Regular surface salt testing, combined with proper surface preparation and coating inspection, helps plants protect their equipment in a planned and measurable way.

It fits naturally into any asset integrity management programme. A regular corrosion assessment that includes coating condition checks and surface cleanliness records gives engineers a clear view of where risks are building up. And when a failure does happen, metallurgy root cause failure analysis often traces it back to something simple, like unremoved chlorides under the paint film. Understanding the cause helps prevent the same problem from repeating on the next job.

The benefits add up quickly:

• Better coating adhesion and longer service life

• Fewer blisters, rust spots, and early repairs

• Lower maintenance costs

• Reduced risk of unplanned shutdowns

• Improved equipment reliability and safety

Conclusion

Salt tests on the surface might look minor, yet they can set the outcome for a coating. Some coatings fail in about two years. Others keep going for far longer. Salts that you cannot easily see, such as chlorides and sulphates, often drive blistering and corrosion under the coating. They can also lead to early breakdown.  

To get useful results, check the surface, use the correct limits, and clean the area again if the process calls for it. Keep clear notes of what was done and what was found. When teams do this, chemical plants can better protect their equipment. It also helps them steer clear of costly surprises.  

Need help with coating checks, corrosion work, or failure review? CorroSafe Consultant can support your team. We work with plant staff to locate what is really behind coating issues. Then we suggest simple steps to reduce the risk. Reach out to CorroSafe Consultant today. Strengthen your asset integrity plan and aim to extend the service life of coated assets.