The cracking of zinc spangles on hot-dipped galvanized steel
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THE importance of hot-dipped galvanized sheet steel to the construction and light engineering industries arises from its resistance to corrosive attack under a wide variety of atmospheric conditions. However under certain conditions some problems occur and one of these, the relatively poor adhesion of paint at sharply bent regions of a sheet, has received considerable attention. The papers of Leidheiser and Kim '-4 and Leidheiser and SuzukP have outlined some of the factors affecting the adhesion of paint to galvanized steel and have given methods of predicting the degree of adhesion. Both were reported to be strongly related to the fracturing tendencies of the grains (spangles) of zinc. Due credit should be given to the unpublished pioneering work of Barlow and Harvey6 who conducted tests on bend specimens from sheets from many sources, as well as laboratory produced coupons. In this and the work of Harvey and othersV.8 and Sebisty and coauthors9,~0 it was established that: 1) There are two clearly distinguishable modes of failures--boundary cracking and intragranular "straight-line" cracking. 2) The boundary cracking occurs after only a small amount of bending and is very prevalent when the spangle size is small. 3) Many of the cracks are too fine to be visible to the naked eye. They widen and become more apparent as bending proceeds. 4)The intragranular cracks are "usually in long parallel groups or in groups of right-angled cracks which are typical zinc cleavage cracks.'6 (Leidheiser and Kim4 established that intergranular cracks formed on (0001) or (1120) planes.) 5) The cracking tendencies of the spangles does not
DON JAFFREY, formerly Senior Lecturer, University of Newcastle, is Head, Department of Metallurgy and Mining, Royal Melbourne Institute of Technology, Melbourne, Australia, 3000, DAVID BROWNE is Senior Lecturer, Department of Metallurgy, University of Newcastle, Shortland, N.S.W., Australia, 2308, and TERRENCE J. HOWARD is Research Officer, Newcastle Works, the Broken Hill Prop. Ltd., Newcastle, Australia. Manuscript submitted January 17, 1980.
depend on the coating weight (i.e. zinc layer thickness). 6) The influence of zinc alloying additions, Sn and Pb, on propensity to cracking were small, with Sn having no detectable effect and Pb having a slightly beneficial effect. 7) Some of these effects could be explained in terms of the influence of the variables on the ultimate tensile strength of the zinc layer. Certain of these features have recently been independently confirmed by Leidheiser and Suzuki ~ in their more extensive study. A problem with both these studies is that the bending test introduces complex stress patterns which are difficult to quantify. The test can not be related to standard methods for assessing the yield / flow / fracture stress of zinc crystals. Our earlier work" and the current investigation was undertaken in part to overcome this deficiency and to study the surface topography and crack initiation aspects of this problem. EXPERIMENTAL DETAILS The cracking of the zinc span
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