Duplex Systems Hotdip Galvanizing plus Pointing

Chapter I Introduction to duplex systems Although belonging to the group of older building and construction materials, steel has remained the most important commodity in to­ days technology. In comparison with other materials, such as con­ crete, stone, wood, etc., steel has a number of both technical and economic advantages — i.e., relatively light weight per unit of vol­ ume of construction, easy repair and addition possibilities, easy formability, wide range of available parts and forms and economical means to assemble parts. Also, from an ecological point of view, steel has attractive characteristics.

F O R E W O R D

It gives me great pleasure to write the foreword to this book because

it is unique in several respects

It is the first comprehensive text ever written on the specialised topic of duplex systems, which is the generic term for painted hot-dip batch-galvanized structural steel Another feature of this book is that both the traditional batch hot-dip galvanizing process and the modern sheet galvanizing processes, such as those used in the automotive and building industries, are covered

Furthermore, unlike many other monographs, it offers a combi­nation of practical information, which will enable the engineer to select the proper materials in a wide range of different conditions, and scientific background information The practical guidelines given in this book are backed up and supported by an impressive amount of technical and scientific discussions or justifications Even modern surface analysis tools are described and recent applications included The literature is covered until very recently and includes the entire world literature on the subject matter

The author of this book has an unsurpassed experience in this field and many of the cited examples of successful (or unsuccessful!) attempts to use duplex systems for corrosion protection of structural steel are drawn from the personal experience that the author has acquired in the past forty years

I can, therefore, recommend this book strongly and without reservation to all engineers in the paint, metallurgical or galvanizing industries who come across painted galvanized steels, whether it be for a cursory look at which paint might do the trick or for a more in-depth understanding of the mechanism of paint failure in a particu­lar environment or application

Prof Dr W.J van Ooij Department of Materials Science and Engineering

University of Cincinnati

Ohio, USA

P R E F A C E

"Save the surface and you save all."

When I wrote my first book on metal coating in 1939 the above slogan

headed the introduction Within the limits of it, this is the basis of the

coating industry and its allied branches The enormous costs of

corrosion, its prevention, as well as reconditioning corroded surfaces

are nowadays well known In industrial countries such costs amount

to 3-4 percent of their Gross National Products

Duplex systems (galvanizing plus painting) are an important

contribution to the corrosion prevention of steel surfaces, which is

based on its synergistic effect

In this book — the first on this subject in English — I have

endeavoured to provide a detailed survey of duplex systems from

their beginning in the 1950s to the systems and their applications

today The development of modern surface analysis has contributed

considerably to an improved knowledge and composition of duplex

systems in the 1990s Many international case histories are mentioned

in the last chapter A special chapter describes failures in duplex

systems, their origins and reconditioning methods

I would like to express my gratitude to all who have provided

information and/or illustrative material They are, in alphabetical

order: AKZO Coatings, Armco Research & Technology, Australian

Zinc Development Association, BIEC (European Producers of Gal­

valume), Cape Galvanising (Pty) Ltd., Centre de Recherches Metal

-lurgiques (CRM), Clements Corrosiepreventie B.V., Dietsche

Holding Co., Duncan Galvanizing Co., Electriciteitsbedrijf

Zuid-Holland (EZH), European Coil Coaters Association, Galvanizers

Association of Australia (GAA), Galvanizers Association of the UK,

Geholit + Wiemer GmbH, GemeinschaftsausschuB Verzinken e.V.,

Hoogovens, International Lead Zinc Research Organisation

(ILZRO), MAVOM, National Association of Corrosion Engineers

(NACE), Nordisk Forzinkningsforening, K.-A Van Oeterenf,

Rem-brandtin Lackfabrik, Schaepmans Lakfabrieken/Delta Coatings BV,

South African Hot-Dip Galvanizers Association, Stichting Doelmatig

Verzinken (SDV), Centre TNO Coatings, VDF Verband der Deutschen

Very special thanks are due to Prof Dr William van Ooij and Frank C Porter for their valuable suggestions and remarks, to Karla

W Nieukerke for her helpful assistance during the writing of this book, Frances Holmes for her meticulous typing of the manuscript, and to Jo and Leon Verstappen for their expert assistance in comput­erizing text and illustrations

I would also like to express my sincere thanks to the publishers, Elsevier Science, for their valuable cooperation and assistance during the publication of this book

Finally, I would like to quote from the 'Immense Journey" by Loren Elseley:

" we cannot in one lifetime see

all what we would like to see

or learn all that we hunger to know "

Jan F.H van Eijnsbergen The Hague, The Netherlands

1994

S P O N S O R S

The author and publisher express their gratitude to the following companies and institutions for sponsoring the colour printing in this book

- American Galvanizers Association (USA)

- Asociacion Tecnica Espanola de Galvanizacion (Spain)

- Associazione Italiana Zincatura (Italy)

- Bammens Groep BV (The Netherlands)

- B.E Wedge Holdings Ltd (UK)

- Billiton Marketing & Trading BV (The Netherlands)

- B.V Verzinkerij Heerhugowaard (The Netherlands)

- Cape Galvanising (Pty) Ltd (Republic of South Africa)

- Galvanizers Association (UK)

- Galvazinc Association (France)

- Geholit + Wiemer Lack- und Kunststoff-Chemie GmbH (Germany)

- Hogeschool Utrecht [HO-U] (The Netherlands)

- Hoogovens Corporate Research (The Netherlands)

- Institut fur angewandtes Feuerverzinken GmbH (Germany)

- International Lead Zinc Research Organization Inc (USA)

- Nederlands Corrosie Centrum (NCC) (The Netherlands)

- Nordisk Forzinkningsforening (Sweden)

- Outokumpu Zinc Oy (Finland)

- Schaepmans' Lakfabrieken/Delta Coatings BV (The

- Verzinkerij Meerveldhoven BV (The Netherlands)

- Weert Groep B.V (The Netherlands, Belgium and

Germany)

I n t r o d u c t i o n to d u p l e x s y s t e m s

Although belonging to the group of older building and construction

materials, steel has remained the most important commodity in to­

day's technology In comparison with other materials, such as con­

crete, stone, wood, etc., steel has a number of both technical and

economic advantages — i.e., relatively light weight per unit of vol­

ume of construction, easy repair and addition possibilities, easy

formability, wide range of available parts and forms and economical

means to assemble parts Also, from an ecological point of view, steel

has attractive characteristics

The annual world production amounts to approximately 800

million tons This tonnage is divided for 1990 over the following

countries or groups thereof [1]

About one-third is made from scrap — thus saving considerably

on the costs of production, as well as on the costs of raw materials and

manufacturing In comparison with a recycling factor of steel of

approximately 90, aluminium has 34, glass 45, paper 40 and plastics

10 [2] Also in the sector of corrosion protection, the need for a more

sparing use of materials and thus, in connection with their coating

systems, a re-assessment of anticorrosive performance versus

recy-clability will be unavoidable The history, present position and pros­

pects of zinc recycling are summarized in a recent brochure of the

European Zinc Institute [17] Modern duplex systems are already

European Community (EC-12)

Other West-European countries

Eastern Europe

North America

South America

Japan

Other Asiatic countries

Other countries (continents)

Ill

Fig 1-2 Composition of rust

Sulphate nests: in industrial

(y-FeOOH) in moist beach areas

fulfilling this combination of characteristics Also, modern research and development in the steel industry has resulted in the use of many special steel types — such as chromium steels, stainless steels, high-strength low-alloy (HSLA) steels and many other alloyed steels which are currently in use for special applications

As with all other constructional materials, steel also has unfavour­able properties (in this case, just one), namely its surface will rust when exposed to the elements Rust is not a single compound: its composition varies with the type and concentration of atmospheric elements and pollutants, as well as with the degree of humidity and the duration of wetness

Rust generally consists of hydrated iron oxides, e.g goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganite (β-FeOOH) and/or magnetite (Fe3U4). In sulphur-containing atmospheres, nests of iron sulphate are present in rust In marine areas, akaganite and salt (NaCl) encrustations are often present in rust [16] See Fig 1-2 The varying types of rust, plus the fact that its volume is roughly twice that of the steel from which it has been formed, are the main technological, economic and ecological reasons for optimally protecting steel surfaces against rust formation and consequential corrosion The corrosion of metal surfaces in general has been published in many handbooks and mono-

C oxygen electrode

C = zone of corrosion by dissolution

Ρ = zone of passivity [ ZnO ]

graphs, a selected number of which, including an atlas [3] on practical

cases of corrosion, are mentioned here in Refs [4-16] Reference is

also made to the publications of the National Association of Corro­

sion Engineers (Houston, Texas, USA) and various corrosion insti­

tutes, associations or study groups in the United Kingdom, Germany,

France, the Netherlands, Scandinavia, Belgium, Italy, Spain, the

former Czechoslovakia, Hungary, South Africa, Australia and Japan

The corrosion of zinc differs markedly from the corrosion (rust­

ing) of steel In the Pourbaix-diagram of zinc the areas of corrosion

by dissolution, the area of passivity (zinc oxide formation) and the

area of immunity are defined, when zinc surfaces are exposed to

distilled water at 25°C See Fig 1-3

Thus hot-dip galvanized steel, galvanized at 440-465°C will,

under these strict circumstances, corrode when the pH-value is above

11 or below 8 In general outdoor practice pH-values of aqueous

solutions below 5.5 or above 12.0 will accelerate corrosion of the

galvanized steel surface Thus the interfacial reactions of (parts of)

the paint film and the zinc (oxide or hydroxide) surfaces will be

different from those of paint films on steel surfaces! It should be

mentioned here that generally the volume of zinc corrosion products

is slightly higher than the volume of zinc from which these corrosion

products have been formed

It has been shown that duplex systems will provide high corrosion protection, as will be discussed in the following chapters of this book

anti-Other constructional materials will also corrode Losses in cor­rosion amount to three to four percent of the Gross National Product

of most industrialized countries [12-14] Apart from the corrosion losses and the economic and ecological consequences involved, it should be mentioned that later repair work and reconditioning do not usually restore the original protective values of the coating system — mainly because the circumstances of reconditioning (local climate, type of structure, surroundings, plant production scheme, etc.) are often not the optimum for reconditioning [15]

In all cases where the selection of an anticorrosive material is impossible for both technical and economic reasons, the use of pro­tective coating systems has been a universal practice The general characteristics of such paint systems are:

(i) when applied to steel surfaces free from rust and impurities they must yield adequate protection for periods between seven and twelve years before rust starts to form;

(ii) they must sufficiently retard the diffusion of moisture, oxygen, water and pollutants;

(iii) they must remain active at the interface steel/coating;

(iv) they must be acceptable from an ecological point of view;

(v) they must be able to be applied under the local environmental laws; (vi) they must be easy to recondition and repair during and after their period of protection

A very wide range of both organic and inorganic coatings is available for the protection of steel surfaces, although it must be borne

in mind that a number of paint and bituminous products today are subject to strict environmental and safety laws Modern anticorrosive paint systems are nowadays largely applied in air-conditioned build­ings in order to obtain optimum application conditions, as well as drying and hardening parameters Airless spraying, electrostatic spraying, roller coatings, fluidized bed coating, hot spraying, applica­tion by cataphoresis, etc., have been developed for application proc­esses on meticulously pretreated metal surfaces

Of the aforementioned coating applications, continuous hot-dip galvanized steel and related galvannealed sheet plus one or more organic coatings (mainly stoving enamels) are increasingly used in the automotive industry as well as in fabrication of "white goods", where lifetime-issues of 5-10 years are nowadays required Their use for buildings, bridges, offshore platforms, transmission towers, street furniture etc requires lifetimes between 20 and 50 years Duplex systems can provide such long-range protection, as described and illustrated in Chapter X (case histories)

coatings) will be described in detail They are known under the general name of duplex systems* (see Chapter II, Refs 1-15) Optimal results with duplex systems are obtained when, after hot-dip galvanizing, the zinc surface is immediately painted, as will

be described in Chapters IV, V and VI Although painting may also

be carried out at a later date on weathered zinc surfaces, practice has shown that such duplex systems often do not offer the ultimate in corrosion protection, because surface and weather conditions often vary and/or are subject to changes The application of paints (both liquid and powder paints) on the premises of the galvanizing plant, in order to realize optimal performance of duplex systems, is increasing worldwide

materi-[3] E.D.D During: Corrosion Atlas A Collection of Illustrated Case Histories Vols 1 & 2 Second, expanded and revised edition Elsevier, Amsterdam, Oxford, New York, Tokyo (1991) [4] L.L Shreir: Corrosion, Vols 1 and 2 George Newnes Ltd., London (1963)

[5] U.R Evans: The Corrosion and Oxidation of Metals, 1st and 2nd supplementary volumes Edward Arnold Ltd., London (1968, 1975)

[6] B.C Graig: Fundamental Aspects of Corrosion Films in Corro­sion Science Plenum Press, New York and London (1991) [7] R Collee: Corrosion Marine Editions Cebedoc, Liege (1975) [in French]

[8] S.K Coburn: Atmospheric Factors Affecting the Corrosion of Engineering Metals Publ STP 646, American Society for Test­ing and Materials, Philadelphia (1978)

[9] K Barton: Schutz gegen atmospharische Korrosion; Theorie und Technik (Protection against atmospheric corrosion; theory and engineering.) Verlag Chemie, Weinheim/Bergstr (1973) [in German]

[10] Κ.-A.van Oeteren: Korrosionsschutz durch stoffe; Band 1 & 2 (Protection against corrosion through coat­ing materials.) Carl Hanser Verlag, Miinchen, Wien (1980) [in German]

Beschichtungs-[11] Kh Baumann: Korrosionsschutz fur Metalle (Protection against corrosion of metals.) Deutscher Verlag fur Grundstof-findustrie, Leipzig (1987) [in German]

[12] T.P Hoar: Report of the Committee on Corrosion and Protec­tion Department of Trade and Industry London (1971)

[13] L.H Bennett et al.: Economic effects of metallic corrosion in the USA Department of Commerce/Bureau of Standards (1978) [14] P.J Gellings: Energy-analysis of corrosion and corrosion pro­tection Procestechniek-W, 32 (1977) [in Dutch]

[15] A.H Roebuck and G.H Brevoort: Coating work costs: com­puter application and inspection Paper presented at Corrosion/

86 National Association of Corrosion Engineers, Houston, USA

[16] J Ruf: Organischer Metallschutz Entwicklung und An dung von Beschichtungsstoffen (Organic protection of metals Development and application of coating materials.) Vincentz Verlag, Hannover (1993) [in German]

wen-[17] European Zinc Institute: Zinc recycling: history, present posi­tion and prospects European Zinc Institute, Germany (1993)

D u p l e x s y s t e m s : definition,

function, history a n d g e n e r a l u s e

Duplex systems on a steel surface are generally defined as combina­tions of a metallic coating (zinc, zinc-aluminium or zinc-iron alloys), followed by one or more coats of paint or powder coatings The metallic layer is applied by hot dipping, whereas the following coat­ings are applied by spraying-, brushing-, roller coating- or fluidized bed-processes

Such combinations of metallic and inorganic coatings have proved to offer optimum anticorrosive properties during exposure to the atmosphere

The five basic functions of duplex systems are described below:

(1) By covering the reactive zinc surface exposed to aggressive

climates — such as industrial, marine or urban or combinations thereof— the speed of corrosion of the zinc surface is drastically reduced because oxidation, attack by moisture and contamina­tion (sulphur compounds, nitrogen oxides, ammonia) are pre­vented by the organic coating or coating system (primer, sealer, topcoat(s)) The combination of a zinc coating alloyed to the steel plus an organic coating possesses a synergistic effect — i.e the protective value of the duplex system is higher than the sum

of the protective values of the zinc and paint coatings separately

On the one hand, the organic coating protects the zinc coating against premature oxidation and, on the other hand, the zinc coating (alloyed to the steel surface) will prevent formation of rust on steel Depending on the aggressiveness of the atmos­phere, the synergistic effect can be expressed by the following empirical formula:

Aiuplex = 1.5 tO 2.3 [Dzinc + ^paint]

in which D is the durability in years of outdoor exposure until

not more than five percent of the underlying steel surface has

rusted The factors D and £> int are the durability factors for

Fig 11-1 Combigram of duplex

systems

a: industrial atmosphere; b: urban

atmosphere; c: marine atmosphere;

d: rural atmosphere

Empirical formula for duplex

systems: Duplex = 1-5 to 2.3 [D z \ nc

+

£*paint/-The ciphers 10-60 are approximate

duration of protection values of

duplex systems

Example: To hot-dip galvanized

steel with 70μm zinc coating, a

paint coating of 100 μ m is applied

In an industrial atmosphere, the zinc

coating offers approximately 9

years' protection The paint coating

applied directly to bare steel will, in

the same type of atmosphere, offer 3

years' protection The duplex system

will protect steel against rusting for

approximately 18 years (lower right

part of graph)

Hot dip galvanized steel Years of average

durability up to appr.5% rust on underlying steel

Ο Ι Ο 2 ( ) 3 0 4 ( ) 5 ( )

the zinc coating and the paint coating, respectively, when di­rectly applied to the steel surface In very aggressive atmos­pheres, the synergistic factor amounts to approximately 1.5, whereas in less aggressive climates the factor 2.3 is used It goes without saying that relative humidity and duration of wetness also influence the synergistic factor See Fig II-1

(2) Improvement of the aesthetic appearance: the original silvery zinc coating surface becomes greyish upon weathering and dif­ferences in colour occur because of differences in local oxidation and hydrolysis of the coating This is especially true of zinc-alu­minium surfaces (e.g Galvalume) Although differences in ap­pearance (colour, gloss) are, in the majority of cases, fairly unimportant on utility structures, there is a growing tendency to combine optimum corrosion protection with a pleasing appear­ance (e.g balconies, staircases, railings, fences, lamp poles, buildings)

(3) Following (2), objects are often required to have contrasting colours in order to enhance visibility for traffic safety (road signs, towers near airports, beacons, lighthouses, cranes, trams, buses, railways)

(4) In contrast to (3), it may be necessary to camouflage objects such

as transmission towers, light poles and military installations to

make them appear less obvious In such cases, paints matching

the surroundings of the object or special camouflage paints are

applied

(5) When objects require a very long duration of protection, because

their surfaces are inaccessible or interruption of plant process or

contamination of goods and products are inadmissible, duplex

coatings are used If, at the end of a long period of protection,

reconditioning has to take place, absence of rust (only a weath­

ered zinc alloy layer remains) permits safe reconditioning with­

out having to rely on doubtful derusting procedures

It is emphasized that almost no reconditioning procedures

can yield the same corrosion protection as the original system,

due to the local circumstances under which reconditioning has

to be carried out and with regard to environmental laws

An important factor which is not always sufficiently observed is the

reliability of coating systems, i.e the chance that all characteristics of

a given coating system are realized in practice

The reliability factor (R) of any coating system can be formu­

lated as the quotient of 100 and the total sum of failures (Σ/7):

The factor F represents the sum of all possible failures or deviations

of a coating or coating system and can be calculated in general terms

from the following parameters: form (volume of object); pretreatment

of metal surface; choice and composition of coating(s); application

techniques; climatic factors during application of coating and possi­

ble combinations of these parameters before, during and after appli­

cation

Duplex systems have been shown to have relatively high R

values when compared with paint systems (applied on blasted or

hand-brushed steel surfaces) and with hot-dip galvanizing (see Fig

100 Σ/7

II-2)

Coating system Reliability factor R Fig 11-2 Reliability values for four coating systems

Hot-dip galvanizing

Zinc-rich primer on Sa2^ blasted steel surface

Classical paint system (4 coats) on manually derusted

steel surface

Duplex system (hot-dip galvanized steel plus primer and

topcoat)

1.0-1.2 0.4-0.6 0.2-0.3 2.2-2.4

Fig 11-3 a Airless spraying of

hot-dip galvanized beams with a

white epoxy/polyamide

(two-component) primer (Clements

Corrosiepreventie)

Fig II-3b Hot-dip galvanized

profiles for transmission towers,

spray-painted with an epoxy iron

oxide/zinc phosphate paint Note

bare bolt holes kept free of paint in

order to ensure full conductivity

after erection of towers For this

purpose PVC or rubber caps are

used, which are removed after

painting (Clements

Corrosiepreventie )

Based on the concepts in the preliminary paragraphs, duplex systems are nowadays increasingly applied to a large variety of objects, e.g in the building industry, for traffic signs, road furniture, offshore installations and electrical industries This increased use has also been promoted by the development and use of modern surface analysis equipment, yielding a vastly improved knowledge of the interface of duplex systems and, consequently, the reactions at the interface and their effects on the protective behaviour of coatings A survey of these methods is given in Chapter VIII

The history of duplex systems is difficult to trace in detail It is evident that duplex systems were in use before World War II, as can

be seen from a number of case histories given in Chapter X However,

it was in the early fifties when a more systematic study of duplex systems began At that time, the name "duplex systems" was given to the combination of hot-dip galvanized steel and organic coatings

Fig II-6 Duplex system on

London's East Dock Railway

(F Porter)

Fig II-7 Duplex system on a

staircase railing in Switzerland

Fig 11-8 Duplex system on figures

in a Dutch kindergarten (SDV)

Originally, such studies were carried out in the form of a comparative

test series with various surface treatments, primers and topcoats on

both general galvanized parts and continuously hot-dip galvanized

sheet Often results were rather contradictory Not until the sixties

was a more stabilized insight obtained in the selection of appropriate

surface treatments and paints It was in the seventies that more

sophisticated research was carried out, aided by the development of

modern electronics and instruments Moreover, the development of

more reliable accelerated weathering tests for a range of products

(structural parts, cars, white goods, etc.) replacing the old and, for

duplex systems particularly, unreliable salt spray tests have made a

better insight into the behaviour and protective value of duplex systems possible Surveys of modern testing methods for duplex systems, in which adhesion/cohesion tests play a prominent role, are briefly presented in Chapters VII and VIII

The following chapters provide condensed information on the hot-dip galvanizing process; the type and reactivity of zinc, zinc-aluminium and zinc-iron alloy surfaces; the mechanical and chemical treatments of such surfaces; the selection of appropriate paint and powder coatings and the testing and analysis methods used with duplex systems Moreover, practical examples of possible failures and their prevention and repair — as well as an international range of case histories of duplex systems — will be given, thus combining practical insight with new developments in this growing sector of the coating industry

The use of duplex systems is manifold, both for extending the durability of galvanized parts and structures and for improved aes­thetics A few examples are: buildings, e.g in (petro)chemical plants, locks, road signs, poles, railings, balconies, transmission towers, agricultural and horticultural structures, mine shafts, railway equip­ment and (on continuous hot-dip galvanized steel sheet) automotive parts Apart from applicating companies, more and more general galvanizers are applying duplex systems on their own premises in specially equipped coating plants

Fig 11-9 Duplex system on a

pedestrian bridge in Australia

(GAA)

Fig 11-10 Flarestack of a large refinery in the Netherlands; partly hot-dip galvanized, partly (upper structure difficult to reach) duplex system (SDV)

Fig 11-11 Duplex system on overhead catenary of Dutch State Railways (SDV)

Figures II-3a and b show airless-sprayed hot-dip galvanized beams for transmission towers to be erected in an aggressive climate See also Chapter VI, Figs VI-28 and VI-29

A selection of literature for further reading is given in Chapter

[2] J.F.H van Eijnsbergen: Twenty years of duplex systems — galvanizing plus painting Proceedings 11th International Gal­vanizing Conference, Madrid (1976)

[3] C.A Garcia and E Padinha: Duplex coatings in Portugal Pro­ceedings 11th International Galvanizing Conference, Madrid (1976)

[4] P Hofer and G Albring: Duplex coatings in Switzerland Pro­ceedings 11th International Galvanizing Conference, Madrid (1976)

[5] J.F.H van Eijnsbergen and B Meijnen: Pylons for a new voltage line were given long-term protection by a duplex system applied at the galvanizing plant Proceedings 11th International Galvanizing Conference, Madrid (1976)

high-[6] J.F.H van Eijnsbergen: Increasing air, water and soil pollution

in W and NW Europe and its impact on the corrosion resistance

of galvanized steel Proceedings 13th International Galvanizing Conference, London (1982)

[71 A Turchi: Painting and plastic coating of hot dip galvanized products Proceedings 13th International Galvanizing Confer­ence, London (1982)

[8] K.A van Oeteren: Feuerverzinkung + Beschichtung = System (Hot-dip galvanizing + coating = duplex system.) Bau-verlag GmbH, Wiesbaden/Berlin (1983) [in German]

Duplex-[9] N.R Short, S.O Agbonlahor and J.K Dennis: The nature of the bond between polyester powder films and zinc coated steel Proceedings 15th International Galvanizing Conference, Rome (1988)

[10] J.F.H van Eijnsbergen: New insights into the interfacial zone

of duplex systems Proceedings 14th International Galvanizing Conference, Barcelona (1991)

14 r

Fig II-14 Duplex system on the steel

structure of a Swiss school building

(Dietsche Holding)

10

Fig 11-15 Principal advantages of

Avoidance of under-rusting, thus preventing premature destruction of paint- coating through voluminous rust

Sealing of pores and small damaged areas in paint-coating by formation of insoluble zinc-salts

Steel surface is sealed off by the system of zinc- and zinc/iron alloy layers, preventing rust formation at the zinc/paint interface

Provided permanent good adhesion has been realised, the paint-film will slowly weather away until the largely intact zinc surface has been exposed After corrosion of the zinc layer the alloy layers offer excellent protection

as follow-up layers

When the paint-film has weathered away only surface cleaning is required before (a) new coating(s) (is) are applied, provided this is carried out well before 5% of the surface shows pinpoints of rust

Excellent edge protection, because of adequate covering of corners, edges, etc by the zinc- and zinc/iron alloy layers

Avoidance of contact corrosion when steel parts, protected by duplex systems, are assembled with parts made of copper, stainless steel, aluminium alloys or parts coated by nobler metals

Because of the synergistic effect optimal and mutual corrosion protection

is obtained

Less reconditioning and repair of coating system after transportation and assembly on building site

[11] R Herms: Warum Duplexsysteme? Vortrags- und

Diskussions-veranstaltung (Why duplex systems? Lecture and discussion

meeting.) GemeinschaftsausschuB Verzinken, Dusseldorf (1987)

[in German]

[ 12] K.A van Oeteren: Mangel an Duplexsystemen und ihre Ursachen

(Praxisbeispiele) Vortrags- und Diskussionsveranstaltung

(Faults in duplex systems and their causes (practical examples)

Lecture and discussion meeting.) GemeinschaftsausschuB Ver­

zinken (GAV), Dusseldorf (1987) [in German]

[13] E Landwehr: Praxiserfahrungen mit Duplex-Systemen bei der

Deutschen Bundesbahn Vortrags- und Diskussionsveranstal­

tung (Practical experience with duplex systems of the German

State Railroads Lecture and discussion meeting.) Gemein­

schaftsausschuB Verzinken (GAV), Dusseldorf (1987) [in Ger­

man]

[14] M Bode: Juristische Betrachtungen zum Korrosionsschutz

durch Duplexsysteme Vortrags- und Diskussionsveranstal­

tung (Juridical considerations on corrosion protection through

duplex systems Lecture and discussion meeting.) Gemein­

schaftsausschuB Verzinken (GAV), Dusseldorf (1987) [in Ger­

man]

[15] Geholit + Wiemer GmbH (1988), Internationales Duplex-Fo­

rum, Karlsruhe (International duplex forum.) [in German]

(a) H.-J Bottcher, Duplexsysteme aus der Sicht des

Verzink-ers (Duplex systems from the galvanizer's point of view.)

(b) M Wilk, Duplexsysteme aus der Sicht des Auftraggebers

(Duplex systems from the client's point of view.)

(c) H Tilmans, Duplexsysteme aus der Sicht des

Beschicht-ungsunternehmens (Duplex systems from the coater's point

of view.)

(d) R Schmidt, Duplexsysteme aus der Sicht des

Beschicht-ungsstoff-Herstellers (Duplex systems from the paint manu­

facturer's point of view.)

(e) R Schmidt, Ubersicht der Beschichtungsstoffe fur Du­

plexsysteme: ihre Auswahl und Eigenschaften (Overview of

coating materials for duplex systems: their selection and

properties.)

(f) W Wolff, Duplexsysteme im Korrosionsschutz ab Werk

(Duplex systems in corrosion protection ex factory.)

(g) J.F.H van Eijnsbergen, Erfahrungen mit Duplexsystemen

aus internationaler Sicht auf Zn- und Zn-Al-Oberflachen

(Experience with duplex systems from an international point

of view on Zn- and Zn-Al-surfaces

Chapter III

H o t - d i p g a l v a n i z i n g a n d allied

p r o c e s s e s

In order to apply duplex systems correctly, it is necessary to know the

general characteristics of hot-dip galvanizing and the related hot-dip­

ping processes, such as Galfan and Galvalume Only then can the

correct choice of pretreatment and coating be safely made in connec­

tion with the material presented in the following chapters

In practice it has been shown that 35-65% of all failures in

duplex systems could be traced back to an insufficient knowledge of

both process and surface reactivity Thus, insufficient adhesion and

other deficiencies of a duplex system resulted after short or longer

periods of weathering

Hot-dip galvanizing is a process in which steel parts are dipped

in molten zinc at temperatures ranging from 440°C to 465°C [2]

Details of both job-galvanizing and continuous-galvanizing proc­

esses are given in the Zinc Handbook [2] and in the Handbuch

Feuerverzinken [25] The merits of painted hot-dip galvanized over

painted steel are given in Fig Ill-1 Galvanizing may also be done at

much higher temperatures (550-560°C) nowadays Chidambaram et

al evaluated the characteristics of coatings obtained at this tempera­

ture range [11] This process {delta galvanizing) is carried out not only

on small articles such as bolts, nuts, screws, etc., but also on larger parts,

up to lengths of approximately 6 m

An important galvanizing process is continuous galvanizing of

steel coils, which are cut into sheets which, in turn, are used for the

manufacture of industrial parts in the building and automotive indus­

tries and also in the manufacture of so-called white goods In all these

processes, the steel surface of the parts is completely covered by a

layer of molten zinc which reacts with the underlying steel and forms

zinc-iron alloy layers

In 1989 approximately 4 million tonnes of steel were hot-dip

galvanized in Europe, whereas in North America, Australia, Japan,

Brazil and South Africa approximately 5.5 million tonnes of hot-dip

galvanized steel were produced The fields of application of hot-dip

galvanizing in Europe are presented in Fig III-2

Fig III-l Comparison of the

gradual weathering of painted

constructional steel and hot-dip

galvanized steel

Painted steel After some time, rust is formed underneath the paint films, which only becomes visible after a longer period

of diffusion of oxygen and water through the paint film

Hot-dip galvanized steel Formation of insoluble zinc salts (zinc patina) at the interface by reactions with oxygen and water No rust is formed Pores in the paint film are blocked by zinc corrosion products

Under-rusting proceeds, followed by loss of adhesion of the paint film because of increased volume of rust products (approximately twice the volume of the steel from which they have been formed)

Spread of rust and further loss of adhesion of paint film; rapid destruction of the coating (system)

No loss of adhesion of zinc coating

Further blockade of pores by zinc corrosion products Partial corrosion

of the eta (η = pure zinc) layer Alloy layers still remain intact

Corrosion proceeds very slowly No rust at the interface Partial corrosion

of zinc layer proceeds Alloy layers still remain intact

Protection by paint film is almost completely gone Complete derusting becomes necessary before applying a new paint coating

Zinc coating system is weathered on part of the steel surface Still no rust; slow weathering of the alloy layers

Very few pinpoints of rust

Reconditioning is possible and easy When loss of adhesion is partial, the

overall protective value of the new paint coating is very restricted

Upon further weathering of the alloy layers, more pinpoints of rust appear, slowly extending into rusty areas

Reconditioning is necessary at this point of weathering

Fig III-2 Fields of application oj

hot-dip galvanizing Constructions in the building industries

Street furniture 14% Agricultural and horticultural industries

15 % Power plants, transmission towers 7% Traffic and transportation

6% Fasteners and other joining parts

In Western and Northern Europe approximately 500 job-galva­nizing plants are established, with a total zinc bath volume of approxi­mately 12 000 m3 Production of continuous hot-dip galvanized steel strip amounts to 7.5 million tonnes in Europe (EEC), 7 million tonnes

in USA and 8.5 million tonnes in Japan The percentages of the main hot-dip galvanizing processes in Europe (EEC), USA and Japan are given in Fig III-3

In industrialized countries the ratio of the tonnages of able steels to those of actually galvanized steels varies between 25:1

galvaniz-Job (general) galvanizing

Continuous (strip) galvanizing

be added, as well as the tonnages of electro-galvanized sheet

and 12:1 In 1992 approximately five million tonnes of steel were

galvanized in Western Europe and it is assumed that approximately

10 million tonnes of general galvanized steel have been produced all

over the world [18] The European automotive market will consume

80 000 Mt zinc over the next few years, of which 30 000 Mt zinc will be

used for galvannealing Consequently, the use of duplex systems may

increase substantially, especially in the building and construction

sectors, the automotive industry, the power industry and the street

(traffic) furniture sector

In the general galvanizing process (Fig III-4), the parts are first

degreased in an alkaline solution and rinsed in hot and cold water

Scale and rust are removed by dipping in a 14% hydrochloric acid solution, often followed by rinsing in water Then the parts are dipped

in an aqueous flux solution containing zinc-ammonium chloride and,

in some cases, sodium fluoride and/or potassium, nickel or cerium chloride The thin-film layer is dried at 75-120°C, depending on the type of flux used, and ensures that the steel surface is free from traces

of contaminants in order to obtain optimum wetting conditions during the subsequent dipping in molten zinc A variation of this process is wet galvanizing, in which the parts pass through a flux layer (present

on part of the surface of the zinc bath) before coming into contact with the molten zinc

In Fig III-5, the general composition of the layers in the system

is shown The total coating thickness depends on:

(a) type and surface roughness of the underlying steel;

(b) the composition of the steel surface;

(c) dipping time;

(d) galvanizing temperature;

(e) mass of the object

Of these parameters, (a) and (b) are the most important

When stresses occur as a result of rapid cooling and/or external forces, the delta-alloy layer may sometimes show small cracks Ac­cording to Foct [17] such cracks will not propagate below a critical size of 5 μπι Coatings with a thin delta-layer are less susceptible to damage than those with thicker delta-layers If such cracks should

Total layer composition eta zinc zeta alloy delta alloy gamma alloy

(boundary layer)

η ζ δ γ

Fig ΙΙΙ-5 General composition of

the layers in the coating system of Type of layer

general galvanized steel*

Hardness and abrasion resistance high very high very high very high

*When reactive steels are hot-dip galvanized, the eta layer is often wholly or partially converted into the zeta layer

On continuously galvanized steel (sheets, coils) the zeta and delta alloy layers are non-existent and the gamma layer is extremely thin (<0.1 μπι)

4 0 0

«-· 300

CO G)

Ο 4 8 1 2 1 6 2 0

D i p p i n g t i m e [ m i n ]

spread to the coating surface, it is recommended that a filler be

applied before proceeding with the duplex system

The relationship between the steel composition, dipping time

and coating thickness is shown in Fig III-6a

No Si and (Si+2.5xP) Coating

Percentages by weight of silicon (Si) and silicon plus phosphorus (Si + 2.5xP)

VG = very good; G = good; F = fair; Ρ =poor

Fig II1-7 Galvanized unkilled steel

Normal composition by a zinc (top)

layer and zeta, delta-1 and gamma

(boundary) layers

Silicon-killed and semi-killed steels will generally yield greater coating thicknesses than unkilled (non-reactive) steels Both silicon-killed steels St 52 show a steep rise in coating thickness within the normal range of dipping times, compared with the two types of mild steel

Dipping time is dependent on the volume (mass) of the object;

it varies mostly between 2 and 8 minutes

In Fig III-6b results of hot-dip galvanizing transmission towers

in France (average of 10 000 tons per year) have been assembled [8] Silicon percentages over 0.04 percent by weight yield good adhesion of paint used on the galvanized tower parts

Adhesion of a zinc coating system diminishes at silicon- and silicon plus 2.5xphosphorus values in higher ranges (nos 3 and 4 and also 6 and 7)

Coating thickness increases at Si-percentages of 0.04 and up­wards and also at (Si + 2.5xP) percentages of 0.17 and upwards

It should be noted that all percentages of silicon and phosphorus are obtained by weight from ladle analysis

A recent study [15] carried out at the University of Wales, under the auspices of the International Lead Zinc Research Organization (ILZRO) has yielded important results as to the influence of Si and Ρ

in galvanized steels Enrichment of the steel surface by silicon, as observed earlier by Sandolin, does not fully explain Si-induced high reactivity It has now been found that the alloying element Ρ reacts

= 20 μπι

synergistically with Si, leading to very thick coatings In a new

galvanizability graph the ranges of P- and Si-percentages in steels and

their influence on zinc alloy coating thickness, when galvanizing at

455°C are shown It is believed that, owing to the very low solubility

of trace elements in the Zn/Fe phases, the composition of the steel

surface is changed by such elements and thus reactivity with molten

zinc is also changed

The micrographs in Figs III-7 and III-8 show the differences in

composition of the coating system for an unkilled and a killed steel

Microhardness (7/v) of the various phases are: eta (70), zeta

(110), delta-1 (350) and steel (150) The very high hardness of the

delta-1 phase manifests itself in a high abrasion and wear resistance

(see also p 24) The thermal coefficient of expansion of the alloy

layers varies from 21.5 to 23 χ lO^x K"1; for the zinc (eta) layer

26X10"6 x K"1, and for the steel 12X10"6 x K1 The delta-1 phase will,

during the cooling period, contract almost twice as much as the steel,

thus being more susceptible to microcracking (see also pp 24 and 31)

In Fig III-9, micrographs of a galvanized steel, containing

0.21% silicon, galvanized at 445,455,465 and475°C for 1,2.5,5 and

10 minutes, respectively, are shown Increases in coating composition

and coating thickness are especially evident at galvanizing tempera­

tures of 445 and 475°C At dipping times of 1 minute differences in

Fig HI-8 Galvanized Si-killed steel Outgrowth of zeta-alloy layer towards the surface of the coating

475°C (885°F)

coating thickness are small

In the zinc bath, small percentages of aluminium, tin and/or nickel are also present Aluminium lowers the viscosity of the molten zinc and also inhibits, to a certain extent, the formation of the zinc-iron alloy layers

Tin promotes the formation of hexagonal zinc crystals in the layer, whereas nickel suppresses zinc-iron alloy formation (espe­cially the formation of the zeta alloy layer by forming a nickel-zinc-iron alloy layer in the outer part of this layer)

The molten zinc in the bath is saturated with 1.3% lead, resulting

from the inter-reaction of the zinc and the layer of molten lead on the bottom of the zinc bath, onto which the dross particles drop before being removed from time to time from the bath

After 3-9 minutes in the zinc bath, the formation of the zinc-iron

alloy layers is almost finished and, upon withdrawal from the bath, the alloy layer system is covered by a thin layer of zinc During the cooling period, the formation of zinc-iron alloy layers will continue

down to approximately 200°C Longer immersion times result in

increased coating thickness and further growth of the alloy layers, however not at the same rates

It has recently been shown [1] that the liquid zeta layer forms a

natural channel for the molten zinc to arrive at the interface of the zeta

and delta layers, thus acquiring there the iron necessary for a rapid

build-up of the zeta layer This phenomenon is often observed when

reactive steels are galvanized, such as silicon-killed steels with

0.04-0.10% silicon and over 0.20% silicon Silicon in (the surface of the)

steel accelerates the growth of the zinc-iron alloy layers which may also

be increased by other steel alloying elements such as aluminium, phos­

phorus, manganese and nitrogen — although in a much smaller way

Annealing the steel in a N2/O2/H2O atmosphere renders its sur­

face inactive Oxidation of silicon during the hot rolling process, or

by specific heat treatment, yields a non-reactive steel surface Also,

vacuum annealing while the millscale is still present is a very good

stabilizing process However, such steel treatments are too costly for

many uses Furthermore, striations and local reactive areas may result

from the cold- and hot-rolling processes The topography of the steel

surface always plays an important role when considering its reactivity

during the hot-dip galvanizing processing

This is also the case with several silicon- and aluminium-killed

steels In contrast to the metallic silvery appearance of the pure zinc

layer, the alloy layers are grey Kozdras and Niessen [3] have found

that (apart from the silicon content) the orientation of the zinc crys­

tals, surface and subsurface oxidation and tertiary elements in the

steel — as well as residual tensions in the steel — also influence

coating formation during batch galvanizing Below 0.01% silicon,

there is no reactivity of this element However, with about 0.1%

silicon, destabilization of layers occurs because of

thermodynami-cally induced instabilities in the intermetallic layers S1O2 does not

exert any influence on alloy formation Secondary Ion Mass Spectros­

copy (SIMS) has shown that surface enrichment by silicon may

increase up to 8%, whereas ladle analysis only gave 0.23% By

plating the steel surface with 1-2 μπι iron or copper, it is made

non-reactive; however, this is impractical for batch galvanizing large

parts Also, oxidation of silicon during hot rolling or by specific heat

treatment yields a non-reactive steel surface A similar effect can be

obtained by vacuum annealing with millscale present New insights

into the iron/zinc reactions and into the influence of silicon and

phosphorus in the steel surface on the coating characteristics during

and shortly after hot-dip galvanizing have been presented by Katzung,

Rittig and Gelhaar [26]

Water quenching, instead of air cooling, will often result in

suppression of alloy-layer growth Today, the dry-galvanizing process

is mostly used; but for certain articles, e.g hollow ware, the wet process

is occasionally used There is no difference in the corrosion resistance

of the coating systems obtained by dry or by wet galvanizing Figs

Ill-10 a/b show a modern job-galvanizing plant A modern centri­

fuge-galvanizing plant for small parts is shown in Fig III-l 1

When galvanizing at 550-560°C the coating consists of the

following phases: gamma and gamma-1, compact delta-1, pallisade

delta-1 and fragments of palissade delta-1 in a matrix of eta phase

(zinc) Particularly with cold-formed products, the cooling sequence

influences the propagation of cracks and flaking (see p 24) Hirn

[22,23] reported that very slow cooling after galvanizing at such high

temperatures results in minimized cracking, consisting only of the

delta-1 phase Fast cooling will confine the growth of the delta-1

phase, leading towards an outermost coating layer of the delta-1

Fig Ill-lla/b Modern automated centrifuge-galvanizing plant for the galvanizing of small parts (Verzinkerij Heerhugowaard)

Fig Ill-12 Corrosion resistance of

hot-dip galvanized steel in various

300 600 900 1200 grams per square metre

Μ = marine climate

Β = beaches

I = industrial areas

IM = industrial coastal areas

1 oz/sq.ft -305 g/m 1 μΐη ~7 g/m

palissade in a matrix of pure zinc However, air-cooling galvanized parts, resulting in complete formation of the delta-1 phase through the whole coating, followed by quenching in water, causes cracking and embrittlement

Hot-dip galvanizing of tubes and pipes is generally done by a largely automated dry-galvanizing process After galvanizing, the inside of the pipes are blown with steam to remove excessive zinc and

to obtain a smooth inner surface Outside-only galvanizing is done by transporting long lengths of pipe through the zinc bath, or by pouring molten zinc on the pipe in an hermetically sealed section of the line

It is well known that the corrosion resistance of galvanized steel

is, grosso modo, proportional to the coating thickness (see Fig 12) The zinc-iron alloy layers, such as the delta layer which is almost

III-entirely formed during the high temperature process at 550-560°C, show an increased corrosion resistance — especially in marine and industrial climates In Chapter IV details are given of the reactions occurring at the surface of galvanized steel Differences in corrosion resistance are also due to local conditions, e.g the microclimate

It is impossible in practice to specify any coating thicknesses outside those mentioned in the various hot-dip galvanizing standards

Reactive steel surfaces yield much thicker coatings than mild steel

surfaces, galvanized under similar conditions [3] Galvanizing at

550-560°C yields coatings which are somewhat thinner than those

obtained at the usual bath temperatures

Outdoor weathering of galvanized steel surfaces proceeds in the

majority of cases at an equal rate, after the first three to five months

of weathering No pitting corrosion will occur Details of weathered

zinc surfaces are given in Chapter IV In general, the surfaces of

galvanized steel will gradually weather under atmospheric condi­

tions However, this is not always the case when galvanized steel is

exposed to soils and different types of water [13]

Only in the final stage, when the zinc and zinc-iron alloy layers

have been largely consumed, will small pinpoints of rust occur and

gradually extend over the surface [2]

In the guidance document "Metal coatings for the corrosion

protection of iron and steel in structures" [24], general information is

supplied on the corrosion of steel in the atmosphere, water and soils,

the design of parts to be galvanized, the welding of galvanized steel

parts, various recommendations for selecting metallic coatings (Zn,

Zn/Al and Al) and for duplex systems

Continuous hot-dip galvanizing of steel in coils (see Fig Ill-13),

starts with cleaning the surface in an oxidizing atmosphere and then

in a reducing atmosphere of cracked gas (a mixture of nitrogen and

hydrogen from gaseous ammonia) Thus, all traces of oxides are

removed before the coil enters the bath of molten zinc with the

exclusion of air To suppress alloy formation, the bath contains

0.16-0.20% aluminium and the lead content is generally below 0.01%

The coil, rising from the zinc bath, is very rapidly cooled by gas jets

and often passes quenching rolls in order to remove the zinc crystals

from the surface (skin-passing), thus obtaining an amorphous, very

Fig IH-13 Modern continuous hot-dip galvanizing line for steel coils (Hoogovens)

Fig Ill-14 Influence of crystal

orientation on adhesion of paints on

continuous hot-dip galvanized steel

A Normal zinc crystals Small zinc crystals

I Skinpassed ; no crystals in surface

5 (0001)

Continuous galvanizing yields zinc layers of a much lower thickness than with general galvanizing The coating consists almost entirely of pure zinc, with a very thin alloy layer at the steel interface Customarily, the coating thickness of continuous galvanized steel is expressed in grams per square metre on both surfaces Since corrosion resistance is largely dependent on the thickness of the zinc layer (see Fig Ill-12), such figures can be misleading because of differences in coating weight on each side! Thus, it is impossible to derive coating thickness from coating weight by dividing the latter by 14 (approxi­mately twice the density of zinc) One-side coatings are available for reasons of weldability, economy and chance of contact corrosion with other metals

Such types of sheet are extensively used in the automotive industries and in the manufacture of white goods A schematic view

of the various structures of zinc coating systems on hot-dip galva­

nized steel is presented in Fig Ill-15

In general, optimum results with duplex systems are obtained

when the zinc coating has a thickness of at least 15 μπι, preferably 20

μιη, so as to offer long-term corrosion protection after the paint film

has weathered away, or on damaged areas of the paint film Very thin

zinc layers of the order of 2-10 μιτι, such as are obtained by electro­

plating, are insufficient for duplex systems, destined to offer a corro­

sion resistance for 15 years or more

Galvannealing of continuously galvanized steel sheet results

mainly in the formation of a smooth, fine crystalline, equally distrib­

uted delta phase, containing 7-11.5% iron The alloy crystals are

much smaller and also more regular in size than the eta (zinc) crystals

obtained with continuously galvanized sheet Galvannealed sheet has

an alloy coating thickness of 6-11 μπι per side Immediately after gas

wiping by gas jets, annealing is done for 10 seconds at 550-650°C It

has also been shown that primers applied by cataphoresis show less

cratering above 275-300 V on continuously galvanized sheet than on

galvannealed sheet at application voltages of 225-250 V Because of

its superior corrosion resistance and excellent paint-film adhesion,

galvannealed sheet is increasingly used in the automotive industry,

although its deep-drawing characteristics are below those of continu­

ously galvanized sheet of the same steel quality [2]

1 2 3 4 Fi&- Hl-15- Schematic structures of

2 Dense zeta-alloy growth; local eta-layer grey-silvery appearance

3 Large irregular zeta crystals, due

to high Si-content in steel surface

4 Local outgrowth of zeta-layer over thin spots of delta-layer

5 Galvannealed 0.5 h at approx 650°C; only delta-layer present

6 Many finely dispersed zeta-crystals, due to phosphorus (>0.04%) in steel

7 Continuous galvanized steel coil

8 High-temperature (550-560°C) galvanized; delta-layer only

γ-layer