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Salt – a challenge for the coating advisor (?) The “salt problem” is an ever-returning challenge for the coatings industry and the personnel involved in it. The present document is an attempt to clarify some of the basic aspects of salt, what it is, why it creates problems for us, and how we can measure it and control it. It is primarily intended for internal Jotun use, but may also be used e.g. in customer relations. What is a salt? A salt is a chemical compound which can conduct electricity when dissolved in water. Upon dissolution the salt splits into charged species, positive and negative ions. A sugar, on the other hand, does not split into ions when dissolved, and a sugar solution does not conduct electric current. Hence, sugar is not a salt (!) What kind of salts exist? A huge number of salts exist; however, for our purpose a relatively small number of salts play a role. The salts with which we shall be concerned typically contain Sodium, Magnesium, Calcium, Potassium, Chloride, Sulphate and Carbonate. (The three latter species are based on Chlorine gas, Sulphur and Carbon, respectively.) Among these, Sodium Chloride, NaCl, is the important one. Why be interested in salts? Salt on the steel surface may be a problem whenever the steel is submerged or subject to atmospheric condensation, e.g. under tank tops. Salts give us problems for two reasons:

a) Salts promote corrosion because they attack the steel chemically, and also conduct the electric current “involved” in the corrosion process. For corrosion to occur electric current must be transported along the steel surface. Salt water will make this transport possible and consequently let corrosion take place.

b) Beneath the coating, at the steel surface, the salt will “attract” water by osmosis, and hence contribute to blistering and corrosion. Salts are not able to move through a paint coating, due to the nature of the coating and the salts. However, water can go through the coating. Thus, when salt particles are present on the steel surface it will soon be dissolved by water coming through the coating, and a strong salt solution is formed on the steel surface. Adhesion loss will follow, and blisters become visible. Because this salt solution at the steel / coating interface is more concentrated than the water outside the coating, more water will move through the coating to decrease this concentration difference. According to practical experience osmotic blistering is more often seen in fresh water situations, because here the difference in salt concentration is greater.

How to avoid or remove salts? In a marine environment it is difficult or impossible to avoid salts. Salts will remain on any surface previously submerged and now dry. Blowing winds will carry droplets of salt water from the sea and onto steel structures above the water line. Salts are best removed by fresh water washing, e.g. water jetting. The water will dissolve the salt and carry it away, and ultimately the steel surface will have a salt content at the same level as the fresh water itself. (Most fresh waters contain some salt, although far less than ordinary sea water.)

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Organic solvents like toluene, white spirit and xylene are not suitable for removing salts. Salts are not soluble in these solvents. Likewise, blast cleaning will have a limited effect, and in most cases only serve to drive the salts into the surface or spread it around a wider area. Experience has shown that sufficient salt will remain on the surface to create problems even after a thorough blast cleaning operation. Power tool cleaning may act in a similar way. How to check that the salts are removed? In practice the only way to measure the remaining salt content on a steel surface is to dissolve the salt by way of water, and then analyse the water. The dissolution and analysing techniques may, however, differ significantly. One will also have to make certain assumptions concerning the type of salt etc in order to limit the work involved. Producing a salt water sample for analysis There are three methods for getting the salt into solution for analysis, of which only the latter two are used in practice:

• Boil the steel structure in distilled water, and analyse the water afterwards. This is the Mayne method, which is not very practical for ship’s steel. The method is described in the Journal of Applied Chemistry, December 1959.

• The SCM400 method, where a patch of paper is soaked in distilled water, and then

pressed onto the steel to dissolve the salt into the paper. Afterwards, the wet paper is analysed for its salt content. Equipment for this method is available through Elcometer Inc.

• The Bresle method, where a glue-rimmed plastic envelope is attached to the steel, and the inside void is filled with distilled water to dissolve the salt. The water is then removed for analysis. This is the preferred method in practice. The Bresle method is specified in ISO 8502-6 “Extraction of soluble contaminants for analysis – The Bresle method”.

Water sample analyses Once the water sample containing the steel surface salt has been obtained it may be analysed to determine the salt concentration, and hence the salt content per square metre steel surface. In principle there are three methods for analysis:

• Boil the salt water until only the salt remains, weigh it and calculate the concentration as the salt weight divided by the salt water quantity prior to the boiling.

• Determine the contents of the individual species by chemical methods, e.g. by titration or atomic absorption.

• Measure the conductivity and convert it into salinity. The latter method, conductivity measurement, is the preferred method in practice, due to its simplicity. However, it is required to have a certain understanding of the calculations involved, in order to get a correct answer.

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After the Bresle extraction has produced a certain volume of water to be analysed, the following steps are to be taken: Measure the conductivity, and subtract the initial conductivity, i.e. the conductivity of the wash water before it is used in the patch. In this way we get the net conductivity, i.e. the contribution from the ship’s hull only, and not the background conductivity from the wash water. It is vitally important to use purified, i.e. distilled or ion-exchanged water, as can be acquired at a chemist’s shop or a drugstore. Use of ordinary fresh water may not give the required difference between the measured conductivity and the background conductivity. The background conductivity should be about 5 µS/cm or lower. The conductivity should be given in microSiemens per centimetre, µS/cm. In order to convert the conductivity to salt concentration the following formula is used:

Concentration (mg/l) = 0.58 * Conductivity (µS/cm) (1) (The factor 0.58 is an empirically based conversion factor which relates conductivity to concentration for sodium chloride solutions). To get from “mg/l” to “mg/square metre” we must look at the patch size and the volume of the water involved. Usually, a Bresle sample size of 15 ml water and a 12.5 square centimetre envelope is used. This corresponds to 12 litres per square metre, and the calculation becomes

mg/l * 12 l/m2 = mg/ m2 (2) Putting (1) and (2) together we find that

Salt content (mg/ m2) = 7 * Conductivity (µS/cm) (3) However, the above given calculations are based on the assumption that all salt is NaCl, which is the “strongest” salt constituent of the sea water. To get a value more in line with practical experience the factor above is normally set to the value 6. Hence the final formula for calculation of salt content based on conductivity becomes

Salt content (mg/m2) = 6 * Measured Conductivity (µS/cm) (4) If another patch size, and/or another water volume is employed, the multiplication factor may have to be changed. However, if using one of the patch sizes as given below, and the given water volume, the multiplication factor will remain the same. The table is taken from ISO 8502 part 6, Extraction of soluble contaminants for analysis – The Bresle method.

Patch no. Patch area, mm2 Total volume Calculation factor in (4)

A-0155 155 1,88 ml 6 A-0310 310 3,75 ml 6 A-0625 625 7,5 ml 6 A-1250 1250 15 ml 6 A-2500 2500 30 ml 6

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It is a matter of fact that the conductivity increases with temperature. Hence, a higher conductivity will, erroneously, give a higher salt content. However, since the conductivity is found as a difference between the clean water and the extraction water values, the error will be cancelled. Therefore, salt determination by the Bresle method may be considered as independent of temperature. Conversion of units In some cases the salt content may be given in micrograms per square centimetre, µg/cm2. For conversion, the following formula applies,

mg/m2 = 0.1 µg/cm2 (5)

Remark: Salt concentration (in a volume of liquid!) can also be given in “ppm”, parts per million. This designation has only meaning for comparing numbers, weights or volumes. Hence, the surface salt content can not be given in ppm, however, the term “milligrams per litre” can be converted to “ppm” as follows:

1 mg/l = 1 ppm (parts per million by weight) (6) On some occasions one wants to express the salt content as “chloride”, not as “salt”. In the case of sea water, where Sodium Chloride is the predominant salt, the following conversion formula applies:

mg/m2 as chloride = 0.6 * mg/m2 as salt (7) The same conversion factor, 6, applies for “mg/l” as well. Acceptable salt levels on steel to be coated Based on experience, Jotun specifies the following maxima for salt contents on steel surfaces prior to coating application:

Ballast tanks: 80 mg/m2 Cargo tanks: 50 mg/m2, but 20 mg/m2 for elevated temperatures Outside hull: 80 mg/m2

These values apply for ships, both at the new building and at the refurbishment stage. In comparison, the NORSOK standard M-501 “Surface preparation and protective coating” specifies max 20 mg/m2 NaCl for “offshore installations and associated facilities”. The NORSOK specification is limited to new steel. For “old” steel one has experienced that it is difficult to obtain as low salt levels as for new steel, due to the surface degradation. For fresh water tanks it is particularly important to have a low salt level, due to the risk of osmosis. However, no fixed acceptance value is given for this. At ambient temperature 50 mg/m2 is OK, but when the coating is expected to be used at higher temperatures, 20 mg/m2 should be used.

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Calculation examples 1. Patch size A-1250, 1250 square millimetres, ref ISO 8502-6. 15 ml wash water. Initial

conductivity 3 µS/cm. Measured conductivity 18 µS/cm. Wash water temperature 29 degrees C. What is the salt content on the steel surface? First we calculate the “net” conductivity: 18 – 3 = 15 µS/cm Conversion to mg/sqm: 15 µS/cm * 6 = 90 mg/m2

2. Patch size A-0310, 3.75 ml wash water. Initial conductivity 1.8 µS/cm. Measured conductivity 11 µS/cm. What is the salt content on the steel surface? ”Net” conductivity: 9.2 µS/cm Salt content: 9.2 µS/cm * 6 = 55 mg/m2

Sandefjord, 7th November 2005 Tormod Svartdal