Bresle method
Encyclopedia
The Bresle method is used to determine concentration of soluble salts on metal surfaces prior to coating application, such as painting. These salts can cause serious adhesion problems after time.

Importance

Salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 is ubiquitous in coastal areas. It can be tasted on the lips after walking on a beach. Salt concentration by weight is about 3.5% in sea water. With spray from waves and by other means, salt gets into the air as an aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

, and eventually as a dust-like particle. This salt dust can be found everywhere near the coast. Salt has the property of being hygroscopic, and this property makes it harmful to coatings.

Salt contamination beneath a coating, such as paint on steel, can cause adhesion and corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 problems due to the hygroscopic nature of salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

. Its tendency to attract water through a permeable coating creates a build-up of water molecules between substrate and coating. These, together with salt and other oxidation agents entrapped during coating or migrating later through the coating, create an electrolytic cell, causing corrosion. Blast cleaning
Abrasive blasting
Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically air, or a centrifugal wheel is used to...

 is frequently used to clean surfaces before coating; however, with salt contamination, blast cleaning may increase the problem by forcing salt into the base material. Washing a surface with deionized water before coating is a common solution.

IMO
International Maritime Organization
The International Maritime Organization , formerly known as the Inter-Governmental Maritime Consultative Organization , was established in Geneva in 1948, and came into force ten years later, meeting for the first time in 1959...

 PSPC (performance standard for protective coatings) regulations set a maximum allowable concentration of soluble salts on a surface to be coated, measured as sodium chloride, of 20 mg•m−2. The maximum amount of salt allowed on a surface prior to coating application is typically determined by the coating supplier and the user, such as a shipyard. Standard values have not been established.

Origin of the Bresle Method

The Bresle method was launched in 1995 in the ISO 8502-6 and ISO 8502-9 standards. The test was developed to measure soluble salt concentration on steel surfaces prior to blasting cleaning and coating. Not only ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

, but also the US Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

, IMO
International Maritime Organization
The International Maritime Organization , formerly known as the Inter-Governmental Maritime Consultative Organization , was established in Geneva in 1948, and came into force ten years later, meeting for the first time in 1959...

, NAVSEA
Naval Sea Systems Command
The Naval Sea Systems Command is the largest of the U.S. Navy's five "systems commands," or materiel organizations...

, and ASTM
ASTM International
ASTM International, known until 2001 as the American Society for Testing and Materials , is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services...

 adopted this method as their standard. The method remains the primary and most flexible test method for soluble salts on metal surfaces.

Principle

The Bresle method uses the difference of conductivity
Conductivity (electrolytic)
The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter ....

 of salts in water, each salt having a characteristic conductivity-versus-concentration relationship. The correlation between concentration and conductivity can be found in “Handbook of Chemistry and Physics”. This relationship is useful only if the dissolved salt is known. Sodium chloride, the main salt in sea water, causes a big increase in conductivity with increased concentration.

A special patch is applied to the surface to be tested, and a specified volume of deionized water is injected under the patch. Any soluble salts present on the surface will dissolve in the water. The fluid is extracted and its conductivity measured.

The conductivity of the collected salt solution depends on the volume of water used and its initial conductivity, and the amount of salt in solution depends on the area of the patch. The calculation of the salt per area is based on increased conductivity but in the IMO PSPC method the salt is calculated as sodium chloride, in the ISO 8502-9 method it is calculated as a specific mixture of salts.

Calculation Factors

Factors are applied to the measured conductivity, depending on what is known or assumed about the salt contamination and various conditions, in order to yield meaningful measurements of contamination. Some of the variables are:
  • salt type
  • volume of solution (in below table factors are calculated with a volume of 15 ml solution)
  • temperature
  • equipment-specific scaling


A common source of error is not knowing the composition of the contamination being measured.
Conductivity µS/cm Conductivity mS/m As chloride µg/cm2 As chloride mg/m2 As sodium chloride µg/cm2 As sodium chloride mg/m2 As mixed salts µg/cm2 As mixed salts mg/m2
1 0,1 0,36 3,6 0,6 6 0,5 5
5 0,5 1,8 18 3 30 2,5 25
10 1 3,6 36 6 60 5 50
20 2 7,2 72 12 120 10 100

The Principle

The solubility in water depends on the type of salt. Sodium chloride can be dissolved in cold water to a concentration of 357 g∙l−1. Not only solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...

differs between salts but also the conductivity. When performing a Bresle method test, not only sodium chloride is dissolved but also all other salts present on the surface. Because it is impossible to predict which salts are present at the surface, an assumption is made in the Bresle method. The term “measured as sodium chloride” indicates that this mixture of salts is interpreted as sodium chloride. Reporting how conductivity is factored is essential when creating a report.

Test Patches

A test patch should be as clean as possible. Contamination of a patch can influences the results significantly.

The ISO 8502-6 standard prescribes in annex A that certified patches shall be used. This annex describes a stress test to ensure patch adhesion and wash ability.

Climate

A soluble salts report should include climate conditions and substrate temperature. ISO 8502-6 requires that the test is done at 23°C and a relative humidity of 50%, with deviations reported and agreed upon by both inspector and customer. During arbitration, absence of these values in a report may render the results invalid.
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