DETERMINATION OF WATER SOLUBLE SALTS

Determination of the Water-Soluble Salt Ref. Nr: 110610 Page 1 of 5

THE BRESLE METHOD

INTRODUCTION

The performance of protective coatings applied to steel can be significantly affected by the condition of the steel surface immediately prior to painting. The key factors influencing the coatings performance are:

Presence of rust and mill scale;

Presence of surface contaminants, such as salts, dust, oil and grease;

Surface profile The International Standards ISO 8501, ISO 8502 and ISO 8503 describe the methods of assessing these factors, while ISO 8504 provides guidance on surface preparation methods available for cleaning steel substrates. However, they do not provide recommendations for the protective coating systems to be used, neither, do they contain recommendations regarding the required surface cleanliness for specific situations. Normally these requirements are provided by the coating suppliers and / or the users, such as yards / applicators and owners. The International Standard ISO 8502 (Part 6 & 9) describe the field method (Bresle method)

of extracting water-soluble contaminants from a substrate and the assessment of the total surface density of various water-soluble salts (mostly chlorides and sulphates) on steel surfaces. However, the individual surface density of different type of salts (chlorides, sulphates, etc.) cannot be determined by this method. Water-soluble salts are not easy to detect (visually) as they are almost colourless and may be present even when the steel substrate is dry blast-cleaned to Sa 3 in accordance to ISO 8501-1 / ISO 8501-2. Water-soluble salts beneath a coating may lead to a chemical reaction resulting in blistering, corrosion and adhesion failure between the substrate and the applied protective coating. Typical examples of steel substrates likely to be contaminated with non-visible water-soluble salts are, but not limited to:

- Structures exposed to or being near by marine environment - Structures exposed to or being near by industrial environment and / or acid rain - Tanks internals, e.g. water-ballast tanks, crude-oil, cargo tanks, process or storage

tanks containing aqueous liquids, etc. - Surface exposed to road de-icing salts

Blasting medium with higher level of water-soluble salts and corrosion products may be a source of surface contamination as well. Although the ISO 8502-9 describes a field method for assessment of the total amount of water-soluble salts, the more aggressive ionic type contaminants (i.e. Cl, SO4 , HCO3, Na+, Ca+, etc.) causing corrosion and blistering are easily dissolved and determined, whereas the less aggressive and not easily dissolved minor part of contaminant will remain un-assessed. Therefore the main attention is given to Chlorides, as they are one of the most aggressive salts and most abundant in the marine environment.

DETERMINATION OF WATER SOLUBLE SALTS

Determination of the Water-Soluble Salt Ref. Nr: 110610 Page 2 of 5

WATER-SOLUBLE SALTS

Any salt can be dissolved in water to a certain extent, depending on the type of salt. Solubility can be defined in three groups:

A salt is soluble if it dissolves in water to give a solution with a concentration of at least 0.1 Moles per liter at room temperature.

A salt is insoluble if the concentration of an aqueous solution is less than 0.001 Moles at room temperature.

Slightly soluble salts give solutions that fall between these extremes.

Water-soluble salt when dissolved will change its form and split in ions. The formed salt solution is electro-conductive (electrolyte).

CONDUCTIVITY

Conductivity depends on the type of salt, the concentration / volume of water used and the temperature. Every salt has a different conductivity constant (characteristic conductivity-concentration relationship) and normally surface contamination does not consist of one single type of salt. Because it is impossible to know / predict the type and concentration of salts present on the substrate the ISO 8502 describes the method for assessment of the total surface density of various water-soluble salts (mostly chlorides and sulphates) on steel substrate and gives interpretation of the amount of salts in mg/m as equivalent to:

- sodium chloride ( NaCl )

- mixed salts

- chloride ( Cl ) Generally conductivity is measured in S/cm (micro-Siemens per centimetre) or mS/m (milli-Siemens per meter).

1 mS/m = 1000 S/m = 1000 S/100 cm = 1000/100 S/cm = 10 S/cm

1 mS/m = 10S/cm and 1 S/cm = 0.1 mS/m

DETERMINATION OF WATER SOLUBLE SALTS

Determination of the Water-Soluble Salt Ref. Nr: 110610 Page 3 of 5

CORELATION BETWEEN CONDUCTIVITY AND SALT CONTENT

The relation between conductivity and salt content depends on a number of factors. Basically, patch size, volume of water used, type of salt are the main variables. As it is impossible to know / predict which salts are present on the substrate, an assumption is made for the analysis of soluble contaminants on the substrate. For example, the term measured as Sodium Chloride indicates that the mixture of salts is interpreted as Sodium Chloride only. The formula for calculating the salt content on a surface is as followed:

SC mg/m = (((( C S/cm / SF ) / 1000 ) x V ) / patch size ) x 10000

Where: - SF is Salt Factor for calculation of each type of salt:

- Sodium Chloride ( NaCl ) - SF = 2 - Mixed Salts - SF = 1.667 - Chloride ( Cl ) - SF = 1.2

- V is the volume of water used in ml - Patch size is the surface area of the patch used [cm2]

Reporting how conductivity is converted to content of water-soluble salt is essential in order to provide meaningful measurements of contamination.

BRESLE METHOD PROCEDURE

The Bresle method as described in ISO 8502-6 is a simple, inexpensive field test using flexible cell in a form of adhesive patches designed to be filled with solvent (distilled water). The method uses the difference of conductivity/concentration relationship to determine the concentration of water-soluble salts on metal surface prior to coating application. The individual surface density (concentration) of chlorides, sulphates, etc. cannot be determined by this method. Factors are applied to the measured conductivity, depending on what is known or assumed about the type of salts present. For the purpose of this document the Bresle method is followed using adhesive patch size A-1250 (12.5 cm2). The test method contains the following steps: 1. Adhere a certified patch to a dry surface and de-aerate. 2. Determine the conductivity of the distilled water to be used (this is the blank). 3. Take 15 ml of the distilled water. 4. Fill the syringe with 3 ml distilled water. 5. Insert the syringe needle in the patch at angle of 30 and inject the 3 ml distilled water. 6. Gently rub the patch for 10-15 sec. and remove as much as possible of the water. 7. Re-inject again and repeat the previous step 6 10 times. 8. Extract the water solution from the patch (as much as possible) and add it to the

remaining volume of 12 ml of distilled water. 9. Determine the conductivity of the nearly 15 ml sample. 10. Deduct the value of the blank. (For further details please refer to ISO 8502-6 and / or PPG PMC Information Sheet 1468)

DETERMINATION OF WATER SOLUBLE SALTS

Determination of the Water-Soluble Salt Ref. Nr: 110610 Page 4 of 5

The following table can be used to convert the conductivity reading into an equivalent amount of water-soluble salts measured as:

Chloride ( Cl ) Sodium Chloride ( NaCl )

Mixed Salts This means that despite the fact that not all salts on the substrate will be in example Sodium Chloride, it is assumed that Sodium Chloride is the only salt present on the substrate.

Conversion table of conductivity [ S/cm ] into water-soluble salt on substrate [ mg/m ]

Conductivity

As Chloride

[ Cl ] As Sodium Chloride

[ NaCl ] As mixed salts

S/cm mS/m g/cm mg/m g/cm mg/m g/cm mg/m

1 0.1 0.36 3.6 0.6 6 0.5 5

4 1.44 14.4 2.4 24 2 20

5 0.5 1.8 18 3 30 2.5 25

10 1 3.6 36 6 60 5 50

16 1,6 5,8 58 9.6 96 8 80

20 2 7.2 72 12 120 10 100

24 2.4 8.6 86 14.4 144 12 120

30 3 10.8 108 18 180 15 150

40 4 14.4 144 24 240 20 200

50 5 18 180 30 300 25 250

60 6 21.6 216 36 360 30 300

70 7 25.2 252 42 420 35 350

80 8 28.8 288 48 480 40 400

90 9 32.4 324 54 540 45 450

100 10 36 360 60 600 50 500

200 20 72 720 120 1200 100 1000

250 25 90 900 150 1500 125 1250

300 30 108 1080 180 1800 150 1500

The formula for calculating the salt content on a surface from the conductivity readings is:

S/cm x CF = mg/m Where: - CF is Conversion Factor for each type of salt:

Sodium Chloride ( NaCl ) - CF = 6

Mixed Salts - CF = 5

Chloride ( Cl ) - CF = 3.6 Note: A common source of error is coming from the fact that the composition of the contamination being measured is unknown.

DETERMINATION OF WATER SOLUBLE SALTS

Determination of the Water-Soluble Salt Ref. Nr: 110610 Page 5 of 5

APPENDIX

The following table gives the PPG PMC requirements regarding the maximum acceptable level of water soluble salts on substrates prior to coating application. The water-soluble salts levels were determined in accordance with the Bresle method using standard Bresle Sampler (patch) A-1250 (12.5 cm). The stipulated figures are based on extensive analytic and laboratory testing work as well as practical experience.

PPG PMC Maximum limits of water-soluble salts on steel substrate prior to coating

Conductivity

As Chloride

[ Cl ]

As Sodium Chloride [ NaCl ]

As mixed salts

Specific areas / exposure

S/cm mg/m mg/m mg/m

4 14.4 24 20 Distilled water

10 36 60 50 Cargo Tanks (Crude oil, CPP, Chemicals, etc.)

16 58 96 80 Immersion salt-water (i.e. Outside Hull,

Splash zone, WBT non-compliant with IMO)

20 72 120 100 Dry Cargo Holds /

Atmospheric marine heavy industry (C5M)

24 86 144 120 Atmospheric (rural areas)

250 900 1500 1250 Maximum level of water-soluble salts in

blasting abrasives

The IMO MSC.215(82) Resolution for Water Ballast Tanks requires that the water-soluble salts as equivalent to Sodium Chlorides should not be higher than 50 mg/m (this is equal to 8.3 S/cm measured by the Bresle method using a standard sampler A-1250). Notes: * These above figures represent the maximum allowable levels of water-soluble salts on

the substrate prior to coating application. Widespread industry testing has shown that lowering water-soluble salt contamination to the minimum attainable levels will optimize the performance of the applied coating system.

* Some projects / clients might have their own requirements, which might be more severe

than the PPG PMC requirements. In such a case the requirements imposed by the client and agreed in the contractual specification will prevail.

* The ISO 8502-6 requires that the test is done at 23C and Relative Humidity of 50%.

However, some conductivity gauges have automatic temperature compensation function. Therefore reference should be made to the conductivity gauge manufacturer instruction manual.