Summary for galvannealed versus galvanized Discussion
Galvannealed versus galvanized is discussed in the context of a customer using G90 galvanized sheet that is being press-brake formed close to a bend line and then powder painted. Kevin Tillotson reports that the galvanized coating is galling in the V die, sometimes not visibly, and later releasing during painting to create ridges and poor paint adhesion compared with galvanneal. Herbert E. Townsend replies that, for equal coating thickness, there is generally little or no corrosion-resistance difference between galvanneal and galvanized; the main exception is that galvanized performs better against red rust at scribes on painted material. He cites an SAE paper for comparative data and notes that galvanneal tends to create more forming problems such as powdering and flaking, especially at higher coating weights, which is why it is seldom used above A60. In a later question, Tom Ripp describes flaking on A60 galvannealed tooling and asks whether oil would help. Stan Snopek says galvanneal is prone to flaking and powdering, and that oil reduces friction; he adds that prephosphated, oil…
Q. I have a customer who is currently specifying G90 galvanized material and powder painting for additional corrosion resistance.This application is causing a couple of problems:1) the part is being press brake formed in a pretty narrow V die due to hole geometry being close to the bend line, and this is causing the galvanizing to gall as it is pressed against shoulders of the V die. At times the galled galvanizing is not visible.2)during powder painting operation, the galvanizing that is galled is releasing from the substrate and causing a ridge of the galvanizing and paint along the V die witness mark on the part. In addition the paint does not adhere as well as it would to galvanneal.Therefore, I would like get some corrosion data showing galvanized versus galvanneal material. The ideal data would include both materials with powder paint applied. The customer application is commercial walk-in coolers.Thanks!
Kevin Tillotson
Hawkeye Industries – Tupelo, Mississippi
2005
A. In general, there is litle or no difference in the corrosion resistance of galvanneal and galvanized coatings of equal coating thickness. The lone exception is that galvanized is more resistant to red rust at scribes on painted material. For data, see M. R. Ostermiller and H. E. Townsend, On-Vehicle Cosmetic Corrosion Evaluation of Coated and Cold-Rolled Steel Sheet, Proceedings of the Sixth Automotive Corrosion and Prevention Conference, P-268, Society of Automotive Engineers, Warrendale, PA, pp 65-83 (1993). Also, you can expect to have more problems with forming galvanneal, including powdering and flaking of the coating. These problems become greater with increasing galvanneal coating thickness. For this reason, galvanneal is seldom if ever used at thicknesses above A60. In my opinion, the only advantage to galvanneal over galvanized is that it is easier to spot weld.
Herbert E Townsend
Townsend Corrosion Consultants, Inc. – Center Valley, Pennsylvania, USA
2005
Q. I am currently running an A60 Galvanealed product as specified by my customers. I am getting flaking that is accumulating on my punch dies. This is resulting in frequent pulling of dies for sharpening and excess burr on the part. I am having the same flaking issue on my V Dies and I have to frequently clean out the dies to avoid wear and tolerance issues. Lastly, my spot welders are accumulating the flaking and causing frequent cleaning. I do my own painting and I am having no problems. So far I have received chem treat dry product. I believe ordering material with a light oil vs. chem treat dry is going to help lubricate the tools and remove some of the flaking? Do you agree? Do you have a better idea?
Tom Ripp
– Fishers, Indiana
January 31, 2013
A. Galvanneal (GA) is prone to flaking and powdering on stamping. Oiling will reduce the coefficient of friction and, consequently, the amount of flaking and powdering. We have customers that order prephosphated GA which has a coating of 200-600 mg dried-in-place zinc phosphate/m2 as well as oil. This latter product shows the least amount of powdering and splitting of any of our GA products.
Stan Snopek
– Hamilton, Ontario, Canada
October 20, 2017
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