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74 Corrosion Control Through Organic Coatings Sharp [8] lists nitrites, amines, and phosphates as common materials used to make inhibitors. He notes problems with each class: • If run-off water has a low pH (5.5 or less), nitrite-based inhibitors can cause the residue to form a weak but toxic nitrous oxide, which is a safety concern for workers. • Amine-based inhibitors can lose some of their inhibitive qualities in lowpH environments. • When using ultra-high pressure, high temperatures at the nozzle (greater than 140°F [60°C]) can cause some phosphate-based inhibitors to revert to phosphoric acid, resulting in a contaminant build-up. In the 1966 edition of the manual Good Painting Practice, the Steel Structures Painting Council recommended an inhibitor made of diammonium phosphate and sodium nitrite [9]. Other possibilities include chromic acid, sodium chromate, sodium dichromate, and calcium dichromate. The 1982 edition of this manual does not make detailed recommendations of specific inhibitor systems [1]. Van Oeteren [10] lists the following possible inhibitors: • Sodium nitrite combined with sodium carbonate or sodium phosphate • Sodium benzoate • Phosphate, alkali (sodium phosphate or hexametasodium phosphate) • Phosphoric acid combinations • Water glass He also makes the important point that hygroscopic salts under a coating lead to blistering and that, therefore, only inhibitors that do not form hygroscopic salts should be used for wet blasting. McKelvie [11] does not recommend inhibitors for two reasons. First, flash rusting is useful in that it is an indication that salts are still present on the steel surface; and second, he also points out that inhibitor residue on the steel surface can cause blistering. The entire debate over inhibitor use may be unnecessary. Igetoft [12] points out that the amount of flash rusting of a steel surface depends not only on the presence of water but also very much on the amount of salt present. The implications of his point seem to be this: if wet blasting does a sufficiently good job of removing contaminants from the surface, the fact that the steel is wet afterward does not necessarily mean that it will rust. 4.3.3 ADVANTAGES AND DISADVANTAGES OF WET BLASTING Wet blasting has both advantages and disadvantages. Some of the advantages are: • More salt is removed with wet blasting (see 4.3.4). • Little or no dust forms. This is advantageous both for protection of personnel and nearby equipment, and because the blasted surface will not be contaminated by dust. © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 75 TABLE 4.3 Chloride Levels Left after Various Pretreatments Pretreatment Method • Precision blasting, or blasting a certain area without affecting nearby areas of the surface, is possible. • Other work can be done in the vicinity of wet blasting. Among the disadvantages reported are: • Equipment costs are high. • Workers have limited vision in and general difficulties in accessing enclosed spaces. • Clean up is more difficult. • Drying is necessary before painting. • Flash rusting can occur (although this is debatable [see Section 4.3.1]) 4.3.4 CHLORIDE REMOVAL As part of a project testing surface preparation methods for old, rusted steel, Allen [13] examined salt contamination levels before and after treating the panels. Hydrojetting was found to be the most effective method for removing salt, as can be seen in Table 4.3. The Swedish Corrosion Institute found similar results in a study on pretreating rusted steel [14]. In this study, panels of hot-rolled steel, from which the mill scale had been removed using dry abrasive blasting, were sprayed daily with 3% sodium chloride solution for five months, until the surface was covered with a thick, tightly adhering layer of rust. Panels were then subjected to various pretreatments to remove as much rust as possible and were later tested for chlorides with the Bresle test. Results are given in Table 4.4. 4.3.5 WATER CONTAINMENT Mean Chloride Concentration (mg/m2) Before Pretreatment After Pretreatment % Chloride Removal Hand wirebrush to grade St 3 157.0 152.0 3 Needlegun to grade St 3 116.9 113.5 3 Ultra-high-pressure (UHP) waterjet to grade DW 2 270.6 17.8 93 UHP waterjet to grade DW 3 241.9 15.7 94 Dry grit-blasting to Sa 2 1/2 211.6 33.0 84 Source: Allen, B., Prot. Coat. Eur., 2, 38, 1997. Containment of the water used for pressure washing is an important concern. If used to remove lead-based paint, the water may contain suspended lead particles and needs to be tested for leachable lead using the toxicity characteristic leaching procedure © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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