Corrosion Control Through Organic Coatings

Ole Øystein Knudsen, Amy Forsgren

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351649223
• Edition: 2
• Topic: Sciences physiques
• Subtopic: Chimie industrielle et technique

Corrosion Control Through Organic Coatings, Second Edition provides readers with useful knowledge of the practical aspects of corrosion protection with organic coatings and links this to ongoing research and development. Thoroughly updated and reorganized to reflect the latest advances, this new edition expands its coverage with new chapters on coating degradation, protective properties, coatings for submerged service, powder coatings, and chemical pretreatment. Maintaining its authoritative treatment of the subject, the book reviews such topics as corrosion-protective pigments, waterborne coatings, weathering, aging, and degradation of paint, and environmental impact of commonly used techniques including dry- and wet-abrasive blasting and hydrojetting. It also discusses theory and practice of accelerated testing of coatings to assist readers in developing more accurate tests and determine corrosion protection performance.

  1 Introduction

This book is not about corrosion; rather, it is about paints that prevent corrosion. It was written for those who must protect structural steel from rusting by using anticorrosion paints.

Selecting an adequately protective coating for a construction can be a difficult task, as has been demonstrated by many expensive mistakes. Some of them are included in this book, since they give us valuable insights about degradation mechanisms and potential threats to our coating. Repairing coatings in the field is usually a lot more expensive than in the yard due to access problems. In addition, the repair coating rarely has the same lifetime as the original coating. Thus, selecting the right coating and applying it correctly the first time is crucial for the life cycle cost of corrosion protection.

A coating is rarely applied for a single purpose only. Decorative properties are important for almost all constructions. An aesthetic appearance signals care and maintenance, and gives us a feeling of safety, in addition to a better visual experience. Other functionalities a paint coating can deliver are signal colors, thermal insulation, friction, nonstick properties, electric insulation, and so forth. Such requirements will put additional restrictions on the coating selection.

1.1      SCOPE OF THE BOOK

The scope of this book is heavy-duty protective coatings used to protect structural steel, infrastructure components made of steel, and heavy steel process equipment. The areas covered by this book have been chosen to reflect the daily concerns and choices faced by engineers who use heavy-duty coating, including

• Composition of anticorrosion coatings
• Waterborne and powder coatings
• Blast cleaning and other heavy surface pretreatments
• Abrasive blasting and heavy metal contamination
• Weathering and aging of paint
• Degradation of paint by corrosion reactions
• Corrosion testing—background and theoretical considerations
• Corrosion testing—practice

You do not need detailed knowledge about corrosion to read this book, but a certain knowledge about the basics of corrosion will be helpful. Corrosion theory will not be explained here; instead, reference is made to other excellent books on corrosion [1,2 and 3].

1.2      TARGET GROUP DESCRIPTION

The target group for this book consists of those who specify, formulate, test, or do research in heavy-duty coatings for such applications as

• Marine vessels and offshore installations
• Onshore and offshore pipelines
• Boxes and girders used under bridges or metal gratings used in the decks of bridges
• Penstock pipes in hydropower plants
• Tanks for chemical storage, potable water, or waste treatment
• Handrails for concrete steps in the fronts of buildings
• Masts for telecommunications antennas
• Power line pylons
• Beams in the roof and walls of food processing plants
• Grating and framework around processing equipment in paper mills

All these forms of structural steel have at least two things in common:

1. Given a chance, the iron in them will turn to iron oxide.
2. When the steel begins rusting, it cannot be pulled out of service and sent back to a factory for treatment.

During the service life of one of these structures, maintenance painting will have to be done on-site. This imposes certain limitations on the choices the maintenance engineer can make. Coatings that must be applied in a factory cannot be reapplied once the steel is in service. This precludes certain organic paints, such as powder coatings or electrodeposition coatings, and several inorganic pretreatments, such as phosphating, hot-dip galvanizing, and chromating. New construction can commonly be protected with these coatings, but they are almost always a one-time-only treatment. When the steel has been in service for a number of years and maintenance coating is being considered, the number of practical techniques is narrowed. This is not to say that the maintenance engineer must face corrosion empty-handed; more good paints are available now than ever before, and the number of feasible pretreatments for cleaning steel in situ is growing. In addition, coatings users now face such pressures as environmental responsibility in choosing new coatings and disposing of spent abrasives, as well as increased awareness of health hazards associated with certain pretreatment methods.

1.3      COATED METAL SYSTEM

Steel is our most important construction metal, and most of this book is devoted to protective coatings for steel. However, other metals are also painted.

Most aluminum alloys used as construction metals are much less susceptible to corrosion than steel, and paint coatings are in most cases applied for aesthetical reasons and not corrosion protection. The coatings applied on aluminum are therefore usually thinner than the coatings applied on steel. Since aluminum has different corrosion properties than steel, most of the pretreatment processes applied are also different. Coatings on aluminum also differ from steel with respect to degradation mechanisms.

The third metal that is important with respect to organic coatings is zinc, that is, zinc-coated steel. The steel is first hot-dip galvanized, electrogalvanized, or thermally sprayed with zinc, typically to get 10–100 µm of metallic zinc on the surface. The zinc coating is then coated with an organic coating. Such coating systems, called duplex coatings, give excellent corrosion protection, and may have extremely long life with a minimum of maintenance. For constructions in corrosive environments that will be used for several decades, for example, road bridges and offshore installations, such coating systems may give lower life cycle costs than traditional paint systems. The latter usually give lower costs in the construction phase, but when the maintenance costs expected during the life of the structure are included in the calculation, the duplex coating system is often favorable.

The organic polymeric coating system may vary in complexity from the simple barrier coating on a blast-cleaned surface to a combination of conversion coating, primer, intermediate coats, and topcoat.

A coating system consisting of three different coats is illustrated in Figure 1.1. The coats are

1. A primer with metallic zinc particles. The function of the zinc is to provide cathodic protection. Being the first coat, the primer also provides adhesion to the substrate.
   

   FIGURE 1.1 Blast-cleaned steel with a three-coat paint system—zinc-rich primer, barrier coat with flake-shaped pigments, and UV-resistant topcoat. The cross section image was taken with an electron microscope. A range of different qualities may be obtained by varying the pretreatment of the metal surface and the coating system (generic type, thickness, and number of coats).
2. The intermediate coat is typically a barrier coating whose main action is to limit the transport of ions to the metal surface. The transport of oxygen and water is also limited.
3. The topcoat is, if necessary, formulated to protect against degradation by ultraviolet (UV) light. In addition, the topcoat will give the required gloss, opacity, and color, that is, the visual appearance. It is also an additional barrier.

Hence, the individual coats have different purposes and vary in formulation in order to give different properties to the coating system. By choosing different types of coats, thicknesses and number of coats, the properties and expected lifetime of the coating system is changed.

REFERENCES

1. Revie, R.W., and H.H. Uhlig. Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering. 4th ed. Hoboken, NJ: John Wiley & Sons, 2008.

2. McCafferty, E. Introduction to Corrosion Science. New York: Springer-Verlag, 2010.

3. Bardal, E. Corrosion and Protection. London: Springer, 2003.

2 Protection Mechanisms of Organic Coatings

In this chapter, we see that there are primarily three mechanisms by which organic coatings protect against corrosion:

1. Stabilizing the passivating surface oxide
2. Cathodic protection
3. Passivating pigments

The barrier and adhesion properties of the coating are not protection mechanisms in themselves, perhaps a bit surprisingly, but that does not mean that these properties are unimportant. On the contrary, they are vital for the longevity of the coating. Poor adhesion or barrier properties results in rapid failure of the coating and loss of protection.

2.1 BARRIER AGAINST OXYGEN AND WATER

Intuitively, we think that organic coatings protect the substrate from corrosion by being barriers against oxygen and water. Organic coatings are barriers against oxygen and water, but they are not im...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgments
• Authors
• Chapter 1 Introduction
• Chapter 2 Protection Mechanisms of Organic Coatings
• Chapter 3 Generic Types of Anticorrosion Coatings
• Chapter 4 Corrosion-Protective Pigments
• Chapter 5 Waterborne Coatings
• Chapter 6 Powder Coatings
• Chapter 7 Blast Cleaning and Other Heavy Surface Pretreatments
• Chapter 8 Abrasive Blasting and Heavy Metal Contamination
• Chapter 9 Chemical Surface Pretreatments
• Chapter 10 Adhesion and Barrier Properties of Protective Coatings
• Chapter 11 Weathering and Aging of Paint
• Chapter 12 Degradation of Paint by Corrosion
• Chapter 13 Duplex Coatings: Organic Coatings in Combination with Metal Coatings
• Chapter 14 Corrosion Testing: Background and Theoretical Considerations
• Chapter 15 Corrosion Testing: Practice