Handbook of Hot-dip Galvanization

Peter Maaß, Peter Peißker, Peter Maaß, Peter Peißker, Christine Ahner

• Publisher: Wiley-VCH
• Year: 2011
• Print ISBN: 9783527323241
• eBook ISBN: 9783527636891
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Hot-dip galvanization is a method for coating steel workpieces with a protective zinc film to enhance the corrosion resistance and to improve the mechanical material properties. Hot-dip galvanized steel is the material of choice underlying many modern buildings and constructions, such as train stations, bridges and metal domes.

Based on the successful German version, this edition has been adapted to include international standards, regulations and best practices. The book systematically covers all steps in hot-dip galvanization: surface pre-treatment, process and systems technology, environmental issues, and quality management. As a result, the reader finds the fundamentals as well as the most important aspects of process technology and technical equipment, alongside contributions on workpiece requirements for optimal galvanization results and methods for applying additional protective coatings to the galvanized pieces.

With over 200 illustrated examples, step-by-step instructions, presentations and reference tables, this is essential reading for apprentices and professionals alike.

1

Corrosion and Corrosion Protection

Peter Maaß

1.1 Corrosion

1.1.1 Causes of Corrosion

All materials or products, plants, constructions, and buildings made of such materials are subject to physical wear during use.

A general overview of different kinds of wear caused by mechanical, thermal, chemical, electrochemical, microbiological, electric, and radiation-related impacts is shown in Figure 1.1.

Figure 1.1 Types of wear of materials.

The technical and economic mastering of physical wear is difficult, since several causes are intertwined and mutually influence each other. The interaction with certain media of the environment results in undesired reactions of the materials that trigger corrosion, weathering, decaying, embrittlement, and fouling.

While mechanical reactions lead to wear, chemical and electrochemical reactions cause corrosion. Such processes emanate from the materials’ surfaces and lead to modifications of the material properties or to their destruction. According to DIN EN ISO 8044, corrosion is defined as:

“Physical interaction between a metal and its environment which results in changes of the metal’s properties and which may lead to significant functional impairment of the metal, the environment or the technical system of which they form a part.”

Note: This interaction is often of an electrochemical nature.

From this definition, included in the standard, further terms are derived:

• Corrosion system: A system consisting of one or several metals and such parts of the environment that affect corrosion.
• Corrosion phenomenon: Modification in any part of the corrosion system caused by corrosion.
• Corrosion damage: Corrosion phenomenon causing the impairment of the metal function, of the environment or of the technical system of which they form a part.
• Corrosion failure: Corrosion damage characterized by the complete loss of operational capability of the technical system.
• Corrosion resistance: Ability of a metal to maintain operational capability in a given corrosion system.

When unalloyed or alloyed steel without corrosion protection is exposed to the atmosphere, the surface will take on a reddish-brown color after a short time. This reddish-brown color indicates rust is forming and the steel is corroding. In a simplified way, the corrosion process of steel progresses and is chemically based on the following equation:

(1.1)

(1.2)

The corrosion processes begins when a corrosive medium acts on a material. Since (energy-rich) base metals recovered from naturally occurring (low-energy) ores by means of metallurgical processes tend to transform to their original form, chemical and electrochemical reactions occur on the material’s surface.

Two kinds of corrosion reactions are distinguished:

• chemical corrosion Corrosion excluding electrochemical reaction,
• electrochemical corrosion Corrosion including at least one anodic and one cathodic reaction.

1.1.2 Types of Corrosion

Corrosion does not only occur as linear abrasion, but in versatile forms of appearance. According to DIN EN ISO 8044, important variants for unalloyed or alloyed steel are:

• Uniform surface corrosion General corrosion occurring on the entire surface at nearly the same rate.
• Shallow pit corrosion Corrosion with locally different abrasion rates; caused by the existence of corrosion elements.
• Pitting corrosion Local corrosion resulting in holes, that is, in cavities expanding from the surface to the inside of the metal.
• Crevice corrosion Local corrosion in connection with crevices occurring in or immediately adjacent to the crevice area, which has developed between the metal surface and another surface (metal or nonmetal).
• Contact corrosion (aka dissimilar metal corrosion) Occurs at contact surfaces of different metals; the acceleratedly corroding metal area is the anode of the corrosion element.
• Intergranular corrosion Corrosion in or adjacent to the grain boundaries of a metal.

The standard mentioned above describes altogether 37 types of corrosion. These types of corrosion result in corrosion phenomena.

1.1.3 Corrosion Phenomena

EN ISO 8044 defines corrosion phenomena by corrosion-causing modifications in any part of the corrosion system.

Major corrosion phenomena are:

• Uniform surface attack A form of corrosion where the metal material is almost uniformly removed from the surface. This form is also the basis for the calculation of the mass loss (g m⁻²) or the determination of the corrosion rate (µm y⁻¹).
• Shallow pit formation A form of corrosion with irregular surface attack forming pits with diameters much larger than their depth.
• Pitting A form of corrosion with crater-shaped or surface-excavating pits or pits resembling pin pricks. The depth of the pitting spots usually exceeds their diameter.

It is very difficult to differentiate between shallow pit formation and pitting.

1.1.4 Corrosive Stress

According to DIN EN ISO 12944-2: All environmental factors enhancing corrosion (see Figure 1.2).

Figure 1.2 The reduction of SO₂ pollution in Germany over the last 20 years led to decisive reductions of the zinc-removal values (cf. Table 1.1).

1.1.4.1 Atmospheric Corrosion

The corrosion rate in the atmosphere is insignificant as long as the relative humidity on the steel surface does not exceed 60%. The corrosion rate increases, especially with inadequate ventilation,

• With increasing relative humidity.
• With condensate occurring (surface temperature < dew point).
• In the presence of precipitation.
• With increasing pollution of the atmosphere which may affect the steel surface and/or be deposited on it. Pollutants are gases, including sulfur dioxide, salts, chlorides, and sulfates. In connection with humidity, deposits like soot, dusts, salts, etc., on steel surfaces accelerate corrosion.

Temperature also, influences the corrosion process. The following criteria are decisive for the evaluation of the corrosive stress:

• climatic zone;
• cold climate;
• moderate climate;
• dry climate;
• warm, humid climate;
• sea climate;
• local climate.

Local climate is defined as what is prevailing within the radius of the object (up to 1000 m). The local climate and the pollutant content are the basis for the classification of atmospheric types.

• atmospheric types;
• room atmosphere;
• rural atmosphere;
• urban atmosphere;
• industrial atmosphere;
• marine atmosphere;
• microclimate.

The microclimate is the climate immediately at an individual component part. The local conditions, such as influences of humidity, dew-point shortfalls, local humidification and its duration, especially in connection with pollutants occurring at the location, have a significant impact on corrosion.

Table 1.1 shows th...

Table of contents

• Cover
• Related Titles
• Title page
• Copyright page
• Preface to the Third German Edition
• Acknowledgment
• Preface to the Second German Edition
• List of Contributors
• 1 Corrosion and Corrosion Protection
• 2 Historical Development of Hot-dip Galvanizing
• 3 Surface-preparation Technology
• 4 Hot-dip Galvanizing and Layer-formation Technology
• 5 Technical Equipment
• 6 Environmental Protection and Occupational Safety in Hot-dip Galvanizing Plants
• 7 Design and Manufacturing According to Hot-dip Galvanizing Requirements
• 8 Quality Management in Hot-dip Galvanizing Companies
• 9 Corrosion Behavior of Zinc Coatings
• 10 Coatings on Zinc Layers – Duplex-Systems
• 11 Economic Efficiency of Hot-dip Galvanizing
• 12 Examples of Use
• 13 Appendix
• Index