Atmospheric Corrosion of Zinc Coated Steel- Results from a Worldwide Outdoor Exposure Program

In the last decades, numerous national or international exposure programs have been launched to collect data on atmospheric corrosion rates and to establish prediction tools for atmospheric corrosion of metallic structures [1]. Most commonly, carbon steel, zinc, copper and aluminum samples are exposed in selected locations for a year or more. Based on weight loss measurements, dose-response functions and corrosivity maps have been compiled [2,3]. A major limitation of most previous studies is that data have been acquired mainly for bulk metals while studies on metallic coatings are rare. Similarly, the evolution of corrosion products with exposure time has been studied for bulk zinc [4] but few studies on zinc coated steel is reported.

This work is a part of a larger research program were steel with different zinc coatings are exposed on a worldwide basis on well characterized exposure sites in Europe, Asia and North America, see table 1. Corrosion rates of HDG were determined after exposure under different climatic conditions. Corrosion rates for HDG after two years exposure have been measred. Uniformity of the coating corrosion and performance at cut edges was evaluated. The effect of sample orientation and sheltering was investigated. Correlation of data on mass loss with environmental data collected at the exposures sites enabled the derivation of dose-response function HDG.

The formation and development of corrosion products was followed by detailed analysis of samples exposed 0.5, 1 and 2 years using XRD, FTIR-spectroscopy and SEM-EDS. The main corrosion products formed on HDG on the different sites are Zn₅(OH)₆(CO₃)₂, NaZn₄(SO₄)(OH)₆Cl·6H₂O, (Zn(OH)₂)₃ ZnSO₄·5H₂O and Zn₅(OH)₈Cl₂·H₂O. The type and relative amounts of different corrosion products were related to the location and type of exposure site but no systematic changes were observed with the time of exposure.

Table 1. Test sites in the exposure program

Code	Name	Country	Distance to Sea(m)
A	Brest	France	<5
B	Bohus-Malmön (I)	Sweden	<5
C	Bohus-Malmön (II)	Sweden	300
D	Ijmuiden	Netherlands	2600
E	Cadiz	Spain	750
F	Jiangjin	China	̴900 km
G	Ostrawa	Czech Republic	̴500 km
H	Dubai	UAE	2000
I	Daytona Beach	USA	100
J	Quindao	China	10
K	Wanning	China	100
L	Bangkok	Thailand	35 km
M	Sattahip	Thailand	10
N	Singapore	SIngapore	10

References

1- D. Knotkova, V. Kucera, P. Boschek: ASTM STP 1421 Outdoor Atmospheric Corrosion, pp.107, 2002

2- A. Mikhailov, J. Tidblad and V. Kucera: Protection of Metals, 40(6),pp 541-550, 2004

3- M. Morcillo, E. Almeida, B. Chico, D. De la Fuente Analysis of ISO Standard 9223

(Classification of Corrosivity of Atmospheres) in the Light of Information Obtained in the Ibero-American Micat, ASTM STP 1421 Outdoor Atmospheric Corrosion, 2002, pp. 59.

4. I. Odnevall, C. Leygraf, Atmospheric Corrosion, ASTM STP 1239, ASTM, Philadelphia, 1995, pp. 215–229