Corrosion resistance of electrodeposited Zn-Cr alloy coatings

V. Chakarova1, Tz. Boiadjieva-Scherzer2, H. Kronberger3 and M. Monev1*

1 Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad G. Bonchev Str., Bl.11,Sofia 1113, Bulgaria2 Centre of Electrochemical Surface Technology GmbH, Viktor-Kaplan-Straße 2, 2700 Wiener Neustadt, Austria3 TU-Vienna, Institute for Chemical Technology and Analytics, Getreidemarkt 9/164ec, A-1060 Vienna, Austria* monev@ipc.bas.bg

IntroductionNumber of studies demonstrate better corrosion protective properties of electrodeposited Zn-Cr alloy coatings on steel in comparison to Zn coatings. The electrodeposition of the Zn-Cr alloy coatings has been optimized for high speed deposition on steel strips for car body application in the automotive industry. In order to obtain the final product, the electrogalvanized steel sheet undergoes further treatments, including application of additional layers in acidic or alkaline solutions.

AcknowledgementThese investigations have been performed with the support of the Austrian Science Foundation FFG and the government of Lower Austria in the frame of the COMET-program.The authors acknowledge voestalpine Stahl GmbH, Linz, Austria for their co-operation and for providing samples used in the present investigations.

ConclusionsThe steady state potential of the alloy coatings is in about 150 mV more positive than that of Zn (in the boundary case of 10 mass % Cr), but it is still sufficiently negative to the steel substrate to provide sacrificial protection. The shift of the potential in positive direction is related to the Cr content in the alloy.The Zn-Cr alloy coatings are corrosion resistant in a wide range of pH. Significant dissolution is observed when the pH falls below 4.0. The dissolution rate increases with further acidification of the solution. In the acidic range the Zn-Cr alloy dissolves with considerably higher rate than Zn. The dissolution rate of the alloys with higher Cr content is higher. Nevertheless, a bit higher amount of Zn is dissolving. The combined effects of partial or complete dissolution of the corrosion protective film at low pH values and the lower hydrogen overvoltage on the alloys determine the observed behavior.

ExperimentalZn-Cr alloy coatings with Cr content within the 3-10 mass % range and a thickness within the 3-7 μm range were electrodeposited onto low carbon steel substrates from an electrolyte, containing 40 g/l Zn, 15 g/l Cr (added as sulphates) and polyethyleneglycol (1g/l), pH 2.0, in a flow cell, flow rate 4 m/s, c.d. within the 80 – 120 A/dm2 range, 40 oC. The Cr content in the alloy coatings was determined by using X-ray fluorescence analysis. The corrosion studies were carried out in 0.5M Na2SO4 solution at temperature of 25 oC. The pH of the solution was varied by addition of H2SO4 or NaOH. Samples with working surface area of 0.785 cm2 were used.Aim

To investigate the corrosion-electrochemical behavior of Zn-Cr alloy coatings with different Cr content in a model corrosion media, in a wide range of pH (pH 1-13).

Zn-Cr 3 %

Zn-Cr 10 %

а)

0 400 800 1200 1600

Tim e / s

-1 .6

-1.5

-1.4

-1.3

-1.2

-1.1

-1.0

E v

s H

g/H

g 2SO

4 / V

SteelZnCr 10 %ZnCr 3 %Zn

0 100 200 300 400 500 600

Tim e / s

-1.50

-1.48

-1.46

-1.44

E v

s. H

g/H

g2S

O4 /

V

2.5 m

5.0 m

7.5 m

Zn

а)

а)

b)

b) c)

c)

b) c)

Zn-Cr 3 %

Zn-Cr 10 %

Ecorr / V RP / kΩ

Zn -1.49 0.4

Zn-Cr 3 % -1.43 3.4

Zn-Cr 10 % -1.36 5.1

Fig. 1. OCP-time relation obtained in 0.5 M Na2SO4 solution, pH 5.9:a) for steel, Zn and Zn-Cr coatings; b) for Zn coatings with different thickness Fig. 2. Potentiodynamic polarization curves in 0.5 M Na2SO4 solution,

pH 5.9 for steel, Zn and Zn-Cr alloy coatings with different Cr content

Fig. 3. SEM images of Zn-Cr 3% and Zn-Cr 10%coatings after potentio-dynamic polarization to a potential after the first anodic maximum

Fig. 4. Dependence between weight loss for Zn-10%Cr after 3 min immersion time and pH of the 0.5M Na2SO4 solution

Fig. 8. SEM images of the surface of Zn and Zn-Cr alloy coatings with different Cr content – before (a) and after corrosion treatment for time duration of 1 min (b) and 3 min (c) in 0.5М Na2SO4 with pH 1.3Fig. 7. Dissolution rate of Zn and Cr from Zn

and Zn-Cr coated samples (working surface area of 0.785 cm2) in 0.5М Na2SO4 with pH 1.3

Fig. 6. Content of Cr in the alloys before and after corrosion treatment for time duration of 3 min in 0.5М Na2SO4 with pH 1.3

0 2 4 6 8 10 12 14

pH

0

2

4

6

8

Tic

knes

s /

m

as deposited

Fig. 5. Dependence between the thickness of Zn-10%Cr after 3 min immersion time and pH of the 0.5M Na2SO4 solution

a) b)

Results

0 2 4 6 8 10 12 14

pH

0.0

1.0

2.0

3.0

Wei

ght

loss

/ m

g

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

E vs H g/Hg 2SO 4 / V

1E-007

1E-006

1E-005

0.0001

0.001

0.01

0.1

1

log

j / A

cm

-2

ZnZn-Cr 3%Zn-Cr 10%Steel