Concept · Surface preparation

Soluble salts and the Bresle test

Why invisible salts under a coating cause blisters, how the Bresle patch and conductivity measure them, and what the inspector records.

Chlorides and sulfates on steel are invisible and survive blasting in the bottom of pits. Under a coating they draw water through the film by osmosis. This osmotic pressure causes blisters and corrosion.

The why

A coating is not a perfect wall. Water vapour and water move slowly through it. The film acts as a thin membrane that lets water pass but holds back dissolved salt.

Salt on the steel dissolves in the first water that reaches it. This makes a strong solution at the steel and a weak one outside. Water moves through the film toward the strong solution to even the two out. The solution grows and builds pressure, and it lifts the film into a blister. The liquid in the blister is salty.

Chlorides also hold moisture. They take up water at a lower humidity than clean steel does, so the steel stays wet longer. See blistering and undercutting.

Immersion service is the hardest case, because the water is always there. For this reason, tank and ballast specifications set the lowest salt limits.

Where the salts come from

• Sea salt carried in the air and settled on the steel.
• Contaminated recycled abrasive or contaminated wash water.
• Steel left in humid, salty air after blasting.
• Old salts in pits under rust, which dry blasting does not fully remove (ISO 8501-1).

In the field: the Bresle test

ISO 8502-6 covers the extraction. ISO 8502-9 covers the conductivity measurement. Do not mix them up.

1. Find the test location in the specification or the ITP. Pick flat steel near welds. A patch on a weld seals badly.
2. Check the water. It must have at least grade 3 purity (ISO 3696). Distilled or deionised water below 5 µS/cm (0.5 mS/m) usually meets this.
3. Measure the blank: the conductivity of the water before the test. Call it γ1. Each batch of patches also needs a blank test.
4. Stick the Bresle patch to the steel. It seals a known area, for example 1,250 mm² for the A-1250 patch.
5. Inject the water with the syringe, which holds at most 10 ml. Draw the water in and out several times to dissolve the salts.
6. Withdraw the extract and measure its conductivity, γ2. Compensate the meter to 25 °C.
7. Take the rise, Δγ = γ2 − γ1. Convert it to a surface density, in mg/m².

If the meter needs more liquid than the patch holds, add clean water and count the dilution. A meter that reads in mS/m must be converted: 1 mS/m is 10 µS/cm.

Worked example

The conversion is ρ = 500 × V × Δγ ÷ A. V is the water volume in ml, Δγ is the rise in µS/cm and A is the patch area in mm². The formula uses a fixed constant from ISO 8502-9.

An A-1250 patch holds 3 ml of water. The blank reads 3 µS/cm and the extract reads 14 µS/cm. The rise is 11 µS/cm.

Check: 500 × 3 × 11 ÷ 1,250 = 13.2 mg/m². With a 3 ml extract on an A-1250 patch, each 1 µS/cm gives 1.2 mg/m², and 11 × 1.2 = 13.2. This passes the NORSOK M-501 limit of 20 mg/m² and the IMO PSPC limit of 50 mg/m².

A rise of 25 µS/cm gives 30 mg/m². That fails 20 and passes 50. If the blank is forgotten, the first example gives 500 × 3 × 14 ÷ 1,250 = 16.8 mg/m², which overstates the salt by 3.6 mg/m².

What the inspector records

Record the location, the patch area, the water volume, the blank and extract readings, the converted value and the limit it was compared with. Report the unit, mg/m². The specification can ask for the result as NaCl equivalent. A report that gives only a meter number is incomplete.

Conductivity gives total soluble salts. It does not identify chloride. Where the specification limits chloride itself, use a chloride-specific method on the extract.

Where a test fails, wash the area with clean fresh water, preferably at high pressure. Let it drain and dry. Test again at the same place, and check the blast grade for flash rust.

Exam traps

• Trap: ISO 8502-9 covers the extraction. The extraction is ISO 8502-6, and ISO 8502-9 measures the conductivity.
• Trap: the water must read below 5 µS/cm. The requirement is grade 3 water, and 5 µS/cm is only a note.
• Trap: the conductivity result is chloride. It is total soluble salts.
• Trap: the extract reading is the rise. The rise is the extract minus the blank.
• Trap: the Bresle patch measures dust. The tape test (ISO 8502-3) measures dust.
• Trap: one limit fits all jobs. NORSOK M-501 gives 20 mg/m² and IMO PSPC gives 50 mg/m².
• Trap: a Bresle test on an intact old coating shows the salts on the steel. It shows only the salts on the coating surface, so remove a small patch of coating first.
• Trap: Sa 2½ means no salts. The grade is a visual judgement only.

How the exams treat it

The CIP Level 1 practical has a soluble salts station worth 5 points. The CIP Level 1 guide names the ISO standards for soluble salts. The CIP Level 2 practical has a surface contaminant testing station worth 10 points. One FROSIO training body lists a Bresle salt test station.

Use it: Surface preparation guide

99 practice items in the members area mention this topic (CIP 1: 34 · CIP 2: 39 · FROSIO written: 26).

Related

• Standards: ISO 8502 · ISO 11124 to 11127 · SSPC-AB 1, AB 2 and AB 3 · ASTM D714 · ASTM D4940 · IMO PSPC
• Defects: Blistering · Undercutting (underfilm corrosion)
• Surface preparation: Surface cleanliness grades · Surface profile (anchor pattern) · Waterjetting and flash rust