23. - 25. 5. 2012, Brno, Czech Republic, EU

INFLUENCE OF THE SURFACE STATE ON HOT DIP ZN GALVANIZING EFFECTS OF IRON

ALLOYS

Dariusz JEDRZEJCZYK1, Maciej HAJDUGA1, Sylwia WEGRZYNKIEWICZ2, Wojciech

SKOTNICKI1

1 - University of Bielsko-Biala, 43-309 Bielsko-Biala, Willowa 2, Poland. E-mail address:

djedrzejczyk@ath.bielsko.pl

2- BELOS-PLP S.A. BU Galvanizing House, Gen. J. Kustronia 74, 43-301 Bielsko - Biala

Abstract

The paper includes comparative analysis of effects got as a result of hot-dip Zn galvanizing of cast iron with

flake graphite grade EN-GJL-250 and of 30MnB4 steel. Samples with diameter φ = 14.7 mm and

thicknesses of 7 mm were being subjected to different kind of surface preliminary treatment: shot-blasting,

sandblasting, etching and high-temperature oxidation. Galvanizing was being conducted in industrial

conditions, in temperature 460 oC, in the zinc bath enriched in nickel, bismuth and aluminium. In the course

of examinations a thickness and a structure of coated layer were being evaluated using both the optical, as

well as scanning microscope. Additionally elements distribution on the layers cross section was analysed

applying RTG examinations. The final effect was verified by corrosion tests in 0.5M solution of Na2SO4, with

application of potentiostat Si 1286. Examinations showed that applying proper treatment of cast iron surface

the corrosion resistance of the coated zinc layer could be close to these ones measured on galvanized steel

samples. The best results were registered after preliminary high-temperature oxidation of the cast iron and

after shot-blasting of steel.

Key words: hot-dip galvanizing, cast iron, steel, high-temperature oxidation, corrosion resistance

1. INTRODUCTION

The zinc anticorrosion protection of Fe-C alloys (cast iron and steel) is commonly used in different

environments: indoor and outdoor atmospheres, chemicals, sea water, soils and other. In comparison to

other typical coatings zinc layer demonstrates higher adhesion and additionally ensures cathodic protection

to the scratched steel. The mentioned properties come from the structure of coated layer. Typical structure of

Zn coating is composed from eta, zeta, delta and gamma phases. For analysis of the zinc-coating structure

created on iron alloys the basis is the Fe-Zn phase equilibrium diagram [1, 2]. This diagram for several

dozen years underwent many changes. About 80 years ago it was proved, that in Fe-Zn diagram occurs

three phases, arising as a result of the peritectic reaction: - Fe3Zn10, - FeZn7, - FeZn13 and iron solid

solution in zinc - . Next, research referred to different forms of - phase existing within different

temperature range (1, ) and with different morphology (C – compacted, P – palisade). Also 2 phase that

is created as a result of reaction between 1 and phases was distinguished [2]. Because gamma (), delta

() and zeta () are harder than steel they protect steel against abrasion. Next, eta () layer is ductile and

provides the coating impact resistance.

Cast iron is more and more frequent protected in similar way like steel because this widely undervalued

material demonstrates sometimes even higher properties than steel (ADI, vermicular cast iron) [3]. Effect

achieved during hot-dip zinc galvanizing of cast iron differs essentially from steel. When the content of silicon

exceeds level expressed by equation: (Si+2,5P) 103= ESi,P (170% for Technigalva process), thick, brittle and

dull-grey coating is created [4, 5]. Excessive coating growth of Zn on steel with Si content in the range 0,03-

0,12% is called Sandelin’s effect [6], when the silicon content is higher than 0,4% observed process is

23. - 25. 5. 2012, Brno, Czech Republic, EU

defined as Guttman-Niessen effect [7]. Moreover, mechanism of zinc layer growth on the high-silicon steel

surface differs from the mechanism of the protecting layer growth on iron casting surface [8](different values

of exponent “n” in exponential rule of the growth rate: =Ktn, where - layer’s thickness; K – growth rate

constant; t – time of treatment; n – constant of the growth rate). Additionally graphite precipitations included

in cast iron can penetrate inside the zinc coating decreasing its continuity and tightness

Surface preparation has the essential influence on the nature of the Zn coating and its corrosion resistance.

The most popular treatment is abrasive blasting (shot and sand blasting) and the chemical treatment. In the

presented work a multistage treatment - combination of these methods is applied. Also the effect of thermal

treatment of the material (heating steel samples to temperature 250oC and high temperature oxidation of

cast iron in the temperature range 850-1050 oC)

on the formation of Zn coating has been evaluated.

So, the aim of presented research that are continuation of previous works [9, 10] was to compare the

properties and verify the corrosion resistance of Zn coatings created in similar way on steel and cast iron

samples and explanation of the influence of surface preparation (sandblasting, shot blasting, heating,

oxidation) on the final results.

2. EXPERIMENTAL

For testing cylindrical samples =14,7mm, thickness h= 7mm with groove lathed at the whole side wall

circumference were turned. Cast iron samples (with flake graphite grade EN-GJL- 250) were also oxidized in

temperature 850o

C, whereas steel samples (30MnB4) were additionally heated in quartz sand to 250 oC.

Samples were sand blasted, shot-blasted, etched, heated, oxidized or multistage treated – (combination of

mentioned methods).

The hot dip zinc coating has carried out in industrial conditions. The chemical composition of galvanizing

bath was as follows: 99,800 Zn; 0,0584 Ni; 0,0546 Bi; 0,0020 Al; 0,0607 Fe; 0,0078 Pb; 0,0028 Sn; 0,0070

Cu; 0,0008 Cd. Process consisted with the following action: degreasing – dirt and oil removal; pickling – rust,

scale and carbon deposit removal; rinsing – hydrochloric acid removal; fluxing – increasing of zinc adhesion

to alloy; galvanizing in temperature 445-455 oC; cooling – decreasing of sample temperature.

Structure was observed using both optical and scanning microscopes. The elements distribution was tested

with application of X-ray analyser “JCXA – JEOL”. Sample’s surface quality was described additionally by

roughness measurements made after scale layer removal - „MAHR” profile measurement gauge with

”Perthometer Concept” software.

Corrosion resistance was evaluated by application of potentiostat SI 1286 that enables registration of

polarization curves in three electrodes system. Before measure start, samples were placed in corrosion

solution – 0,5M Na2SO4, in temperature 25oC, within 24h. Samples were subjected to polarization in the

same solution, from potential 1000mVNEK in anode direction, with rate 1mV/s.

Results of treatment were evaluated basing upon the following parameters: roughness, surface topography

and corrosion resistance measured by potentio-dynamic method.

3. ANALYSIS OF RESULTS

Chemical composition of materials used in experiment is presented in Table 1. There are essential

differences between chemical composition of applied steel and cast iron mainly with reference to the

contents of C and Si. It can influence considerably the thickness and structure of created Zn coating.

Thickness of zinc layer that was created at the surface of tested alloys is stable and reaches 70-100 µm.

Moreover these higher values were measured on cast iron surface especially when high temperature

oxidation was applied – because the outside surface is more developed – see Fig. 1.

The quality of obtained surface changes in dependence on parameters of surface treatment (Ra= 1,30-4,60

µm). The greatest surface roughness - Ra = 4,60μm was got for cast iron samples turned and oxidized

whereas the lowest values was measured for etched steel (Ra=1,30 µm), Fig. 2.

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Tab.1. Chemical composition of materials used in the experiment

a b

Fig. 1. Zn coating on the cast iron surface after oxidation – a, and on the steel surface - b

Fig. 2. Comparison of measured roughness values of steel and cast iron surface after hot-dip galvanizing,

labelling acc. Tab. 2

Mechanism of Zn growth at cast iron surface is quite different than this one observed in Armco iron (1///)

or steel case (Fig. 3). It follows first of all from different chemical composition of alloy basis (higher silicon

Source

of data

Chemical composition [% wg.]

C S Mn P Si Cr Cu B N Ni Ti Fe

Acc

standard

30MnB4

0,27-

0,32

≤

0,30

0,80-

1,10

≤

0,025

≤

0,025

≤

0,3

0

≤

0,25

0,0008

– 0,005 - - - rest

Acc.

Analysis

30MnB4

0,3 0,11 0,96 0,022 <

0,005

0,2

2 0,044 0,0035 0,007 0,035 0,041 rest

EN GJL

250 3,32

0,03

5 0,55 0,065 1,80 - - - - - - rest

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content and in consequence higher value of equivalent E – see Table 1) and graphite precipitates existing in

the structure. From one side we can state that, in coating forming at the crude cast iron surface only small

task fulfils Z phase (in comparison to coatings at the surface of steel with high value of E equivalent). From

the other side, just Z phase is the dominating phase in coating growing at cast iron machined surface. At the

low zinc coating time phase doesn’t form at all, and graphite precipitations cause irregularity of zinc coating

structure.

a b

c d

Fig. 3. The change of main elements concentration at the cross section of Zn layer: a – Zn, c – C, Fe; b,d –

observed microstructure with marked analysis points

It results from the data presented at Fig. 4 and Table 2 that method of surface preparation before hot-dip Zn

galvanizing exerts significant influence on measured corrosion parameters: corrosion current density icorr and

potential of cathode-anode pass EK-A. Cast iron was turned (T), polished (P), sand blasted (S), shot-blasted

(SH) and multi-stage treated: turned and oxidized (T+O); polished and oxidized (P+O). Steel samples were

additionally coated only after etching (E) and multi – stage surface preparation combined with preheating to

250 o

C (H). The lowest value of icorr= 8,39.10

-9 A/cm

2 was measured for steel being shot-blasted and heating

before coating. A little bit higher corrosion current density was achieved for steel after sandblasting and

heating. When etching was applied icorr values increased even about few orders of magnitude.

In the cast iron case the lowest Icorr values were measured for samples being sandblasted, shot-blasted and

oxidized. These values are very close to corrosion resistance of Zn coating on steel. It means that quality of

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coating on cast iron is similar to steel protection and will be difficult to improve it. The achieved results are

caused mainly by graphite particles removal from cast iron subsurface layer. Voids created in this way are

filled partially by Zn, the created coating is more compact and stronger bounded with metallic matrix.

Fig. 4. Polarization curves registered in the case of cast iron with flake graphite

Tab. 2. Measured parameters of corrosion process.

Sample no. Kind of treatment EK-A [mV] icorr [A/cm2]

Corrosion rate

[m/year]

1 (c.i.) T -1317 2,35×10-7

2,75

2 (c.i.) P -1306 4,59×10-7

5,37

3(c.i.) S -605 8,88×10-8

1,03

4(c.i.) SH -664 9,45×10-8

1,10

5(c.i.) T+O -520 2,19×10-8

0,26

6(c.i.) P+O -422 2,86×10-8

0,33

11(s.) SH+H -386 8,39.10

-9 0,10

22(s.) S+H -502 1,27.10

-8 0,15

33(s.) SH+E+H -1138 8,73.10

-6 102,14

44(s.) S+E+H -1050 1,05.10

-4 1228,50

55(s.) E+H -1185 1,78.10

-5 208,26

66(s.) crude surface -1105 1,25.10

-5 146,25

3* SH+E -548 1,43.10

-8 0,17

1 - S. TURNED

2 - S. POLISHED

3 - S. SANDBLASTED

4 - S. SHOT-BLASTED

5 - S.TURNED AND OXIDIZED

6 - S. POLISHED AND OXIDIZED

I(A

/cm

2)

-2 -1 0 1 10 -10

10 -9

10 -8

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2

10 -1

E (Volts)

1

2

3

4 5

6

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4. CONCLUSIONS

1. Mechanism of Zn growth at cast iron surface is quite different than this one observed in Armco iron

(1///) or steel case. It results first of all from different chemical composition of alloy basis (higher

silicon content and in consequence higher value of equivalent E) and graphite that precipitates in the

structure.

2. The dominating factor deciding on the quality of coating created at cast iron surface after machining is

presence and form of graphite precipitations (chemical composition exerts lower influence).

3. Considering achieved results we can state that using proper surface treatment of cast iron the

corrosion resistance of created Zn coating could be close to these registered for galvanized steel.

4. Method of surface preparation before hot – dip Zn galvanizing influence essentially the corrosion

resistance of Fe-C alloys. The best result was achieved after preliminary high-temperature oxidation of

the cast iron and after shot-blasting of steel.

LITERATURE

[1] HONG H.,N. SAKA H., Transmission electron microscopy of the iron-zinc intermetallic phase, Scripta

Materiala, 36, 1997, 1423-1426.

[2] HONG H. N., SAKA H., Plasticity and grain boundary structure of 1K and 1P intermetallic phases in the Fe-Zn

system, Acta Metallurgica, 45, 1997, 4225-4230.

[3] GUZIK E., Structure and mechanical properties as well as application of high quality vermicular cast iron,

Archives of Foundry Engineering, 10, 3, 2010, 95-100.

[4] KOPYCINSKI D.,GUZIK E., WOLCZYNSKI W., Creation of the zinc layer at the surface of nodular cast iron

industry fittings, Materials Engineering 4, 2006, 1081-1084 (in Polish).

[5] GLADMAN T., HOLMES B., PICKERING F.B., Journal Iron Steel Instr., 211, 1973, 765-772.

[6] SANDELIN R. W., Galvanizing characteristic of different types of steel, Wire and wire product, 15, 1940, 655-

660.

[7] GUTTMAN H., NIESSEN P., Reactivity of silicon steels hot-dip galvanizing, Canadian Metallurgical Quarterly,

11, 1972, 609-614.

[8] KOPYCINSKI D.,GUZIK E., WOLCZYNSKI W., Coating Zn formation during hot-dip galvanizing, Materials

Engineering 4, 2008, 289-292.

[9] JEDRZEJCZYK D., HAJDUGA M., Effect of the surface oxidation on the hot-dip zinc galvanizing of cast iron,

Archives of Metallurgy and Materials, V.56,2011, 839-849.

[10] JEDRZEJCZYK D., Effect of high temperature oxidation on structure and corrosion resistance of the zinc

coating deposited on cast iron, Archives of Metallurgy and Materials, V.57,2012, 145-154.