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C-1 Training: Detection of Soluble Salt Contamination A P P E N D I X C This presentation covers issues related to detection of soluble salt contamination on steel highway structures. It includes several inter-related sections. The reader should be familiar with the introduction to soluble salts training before viewing this material.
C-3 A variety of soluble salt detection methods are currently available, all of which extract salts from a surface by dissolving them into an aqueous electrolyte and then characterizing the soluble salt content of the electrolyte. Characterization may be performed through conductometric or ion detection methods.
C-4 It is more difficult to extract salts from pack rust, crevices, and densely rusted surfaces than âcleanedâ or flat surfaces. A helpful analogy that most people are familiar with is dissolving sugar into a beverageâ¦temperature, amount of sugar, agitation, and other factors influence its dissolution into the beverage.
C-5 Tests shown from top left clockwise: â¢ Soluble salt meter (magnetic meter on flange) â¢ Latex sleeve being used to extract salts from a surface â¢ Adhesively bonded latex patch â¢ Saturated filter paper extraction (black magnetic film holding saturated adsorbent paper on a surface)
C-6 Clockwise from Top Left: â¢ Qualitative detection of ferrous ions using potassium ferricyanide paper â¢ Chloride ion detection tubes â¢ Conductivity measuring instrument for saturated filter paper â¢ Chloride ion paper test strips â¢ Ion-specific test kit for chloride, sulfates, and nitrates â¢ Pocket conductivity meter for evaluating extraction fluid
C-7 There are a variety of signs of salt contamination that are as important as any measured value. For example, white precipitate on surface, rustback after abrasive blasting, and geometrical complexity such as crevices or pitting are all indicators that salt may be present. Shown from Right to Left: a primed, pitted surface; a blasted, pitted steel surface; white precipitate around rusting galvanized steel
C-9 There are various ways to measure salts on surfaces â¢ Multi-step measurements of conductivity are made with a separate conductivity meter, not attached to the retrieval device. â¢ Multi-step measurements of Specific Ions are completed through the following methods: Chloride Ion Paper Strip Test Chloride Ion Detection Tube Chloride Ion Detection by Colorimetric Methods Nitrate Test Strip Sulfate Analysis by Colorimetric Methods â¢ Fully Automated Methods use integrated conductivity meters to measure the conductivity of tested solutions. â¢ Potassium Ferricyanide paper (qualitative method of detecting ferrous ion) and boiling water extraction (lab technique) are less common.
C-12 Photographs from ISO 8502-6 Preparation of steel substrates before application of paints and related products â Tests for the assessment of surface cleanliness â¢ Use sufficient water to immerse conductivity sensor; ~25% of which is used for extraction â¢ 8502-9 Clarifies 8502-6 by specifying 10, 1-minute contact periods
C-13 Photograph reconstructed from page 29 in PPI NBR 63101-000 Preservation Process Instructions Core â¢ U.S. Navy clarified what they perceived to be ambiguities in the state of the art â¢ Specified digital conductivity meter such as the Horiba Twin-Con â¢ Specified 3 ml of deionized water and 10-15 seconds of ârubbingâ to encourage dissolution
C-14 Photo: Meter magnetically attached to surface
C-15 The constant 1.2 is related to calculations based on the assumed combination of salt ions in the tested solution contained in ISO 8502-9. The relationship between conductivity and concentration are ion specific, so for these meters to return a chloride reading, they are assuming a specific mix of ions in the solution being tested.
C-16 Photo: Filter paper applied to surface
C-19 Nonferrous salts are any salts not bonded up with an iron ion
C-20 Key point is that soluble salt contamination may result in visible rustback or leave other residue. The methods presented herein are generally for detection on non-visible soluble salts.
C-22 Key point is that the conversion from conductivity to surface salt density (e.g., âtotal saltâ) is not an exact conversion. Such a conversion would require speciating all of the soluble ionic species on each tested surface (an impractical solution). The ISO 8502-9 conversion was reportedly developed to represent âtypicalâ salts and has proven to be a pragmatic solution many industries.
C-23 Key point is that the relationship between conductivity and âsaltâ depends on what âsaltâ you assume and the geometrical details of your sampling method. The graph contains the theoretical relationship between conductivity and ion concentration for a surface contaminated with Sodium Chloride: 1. Top solid line: solution conductivity vs surface concentration of NaCl 2. Bottom solid line: solution conductivity vs surface concentration of Cl ion Note that this relationship is dependent on the surface area sampled and volume of extraction fluid â the lines are only valid for 12.5 sq cm of sampled surface and 3 mL extraction fluid. The dashed line is the ISO 8502-9 relationship for this surface area and fluid volume.
C-24 Similar to the last slide, this slide reinforces the point that the relationship between conductivity and âsaltâ depends on what âsaltâ you assume and the geometrical details of your sampling method. The ANSI/NACE SP0508 salt is a mixture of 24.3% Na2SO4, 22.1% NaNO3 and 53.6% NaCl. The graph contains the theoretical relationship between conductivity and ion concentration of a surface contaminated with the salt mixture in three scenarios: 1. Top solid line: solution conductivity vs surface concentration of all ions (Na, SO4, NO3, and CL) in ANSI/NACE salt solution 2. Center solid line: solution conductivity vs surface concentration of NaCl 3. Bottom solid line: solution conductivity vs surface concentration of Cl ion Note that this relationship is dependent on the surface area sampled and volume of extraction fluid. The dashed line is the ISO 8502-9 relationship for this surface area and fluid volume.
C-25 The amount of surface salts that can be measured by the meter vary due to the limitations on the slide.
C-26 Photos show: 1. A rubber magnetically adhered patch leaking due to incomplete seal on the surface 2. A latex adhesively bonded latex patch cell leaking due to incomplete seal on the surface 3. The effect of evaporation from saturated filter paper on the detected salt value
C-27 Picture on the left shows a roughly pitted surface after a fully automatic conductivity test was performed. Extraction fluid did not fully saturate the surface inside the area and leaked from the cell. Picture on the right displays the leakage of a latex sleeve, which leaked due to a poor seal. On pack rust surfaces, the solution will leak down through the rust and wet out the sides of the cell being tested. On rough surfaces the patches will not adhere, and have difficulty getting down into the pit areas where salts are likely to be in a higher concentration.
C-28 The orange bars represent surfaces that have been aged for a few days at high humidity after doping whereas the blue bars represent tests done immediately after applying salt to the surface. Both chloride ion methods and conductivity methods show a decrease in extracted salts. The same phenomena is not observed on a polymer coated surface, suggesting that there is an interaction between the steel and salt that decreases its solubility.
C-29 Although the descriptions of using many devices are simple, there are many small details that are important to keeping all devices functioning properly.
C-30 Small variation measurements: As close as possible together, without going over other readings or causing cross contamination Useful for checking unique surfaces. In photograph, checking if the run off contained chloride salts Larger variation Making readings farther apart on the surface, a larger variation in surface contamination can be expected Useful for checking how surface contamination may change across testing area Note: Readings in photo are taken across a span of about 4 feet wide. In photograph, detection was tried directly on pitted surface but leaks occurred, so flat surface directly next to pitting was measured. This is not a substitute for the pitted area, but the next best option.
C-31 Two sides of the same problem: Conductivity may be identifying contaminants that wonât negatively contribute to paint adhesion and performance Chloride may be missing some contaminants
C-33 A good reference document that is included in the Guidelines Bolded sections were discussed previously in the section on detection methods
C-34 Not necessary for the average person to understand in detail, but they should know that this standard is used to determine if alternative test methods are providing relatively meaningful readings.