Hot Dip Galvanizing

Date 04-27-2004 12:41

Put galvanizing in Google and you will get many sites to explain the process. The AGA will provide much knowledge. http://www.galvanizeit.org/

By swnorris (****)

Date 04-27-2004 12:46

Hi Brian,

Here's what I've read: Hydrogen embrittlement affects succeptible steels, which are those in the higher strength range. This form of embrittlement is caused by hydrogen ions migrating into the steel through the grain boundaries. The hydrogen ions come from the acid, so it is the pickling process that determines the liklihood of hydrogen embrittlement occurring. Steels with tensile strengths exceeding 750 MPa are succeptible to hydrogen embrittlement and it is critical that this issue be identified, as such steels are usually used in critical load applications. Hydrogen embrittlement can be avoided by using other pre-treatment methods such as abrasive blasting, instead of acid pickling. It can also be dealt with by post treatment with appropriate products such as high tensile bolts in grades 8 and 10 by heat treating the items after galvanizing.

By swnorris (****)

Date 04-27-2004 13:09

Brian,

A great source of information is the ASTM standard on hot dip galvanized coatings. I have a copy of an old standards from 1977. One of the standards is ASTM A 123, which is the standard for galvanized coatings on products fabricated from rolled, pressed, and forged steel shapes, plates, bars, and strip. Section 6 addresses steel embrittlement in general and states in part the following: "Steel that is cold worked may become embrittled, depending on such factors as the type (strength level, aging characteristics), thickness, and degree of cold work. Galvanizing may accelerate embrittlement. The tendency of embrittled steels to fail in a non-ductile manner is greater the lower the temperature at which the steel is used. Therefore precautions against embrittlement shall be taken by the designer, the fabricator, and the galvanizer". Also, ASTM A 143 is the standard for safeguarding against the embrittlement of hot dipped galvanized structural steel products and the procedure for detecting embrittlement. It states in part in 3.2: "In the case of galvanized steel, the acid pickling reaction prior to galvanizing presents a potential source of hydrogen. However, the heat of the galvanizing bath partially expels hydrogen which have been absorbed. In practice hydrogen embrittlement of galvanized steel is usually of concern only if the steel exceeds approximately 150 ksi (1100 MPa) in ultimate tensile strength, or if it has been severely cold worked prior to pickling." Also, in 6.1 it states: "Hydrogen can be absorbed during pickling and in some instances as noted in 3.2, may continue to contribute to embrittlement of the galvanized product. The liklihood of this, or of surface cracking occurring, is increased by excessive pickling temperatere, prolonged pickling time, and poor inhibition of the pickling acid. Heating to 300 degrees F after pickling and before galvanizing in most cases results in expulsion of hydrogen absorbed during pickling." I'd be happy to send you a copy of these two standards by fax if you don't have access to them.

By jwright650 (*****)

Date 04-27-2004 16:46

LBE is another method of embrittlement that has been suggested to cause cracking in galvanized steel.
John Wright

By swnorris (****)

Date 04-27-2004 19:03

Hi John,

I don't think I've heard of LBE, but another type of embrittlement that can arise from the pickling process is strain age embrittlement, which is one of the most common causes of brittle failure in steels, and is caused by severely cold working the steel by forming, bending, or punching. The steels involved in the galvanizing process do not contribute to stain aging failure, but rather the heat of the process accelerates it. The aging of the strained steel will proceed to brittle failure over time, just as the name suggests. The aging process can just as easily be progressed through localized heating by welding or gas cutting. When steel is cold worked, the crystal lattice in the stressed area is severely deformed and carbon atoms migrate to the most stressed locations, raising the hardness and tensile strength of the steel in the stressed area. Heating the steel to the transition temperature (around 1292 degrees F), hot bending the steel, or ensuring that bend radii are not too sharp, will avoid strain aging problems. Strain aging is also commonly encountered on thick plates that have punched holes. To remedy this, the holes can be either drilled or sub punched and reamed, which will remove the highly stressed metal around the hole.

By swnorris (****)

Date 04-27-2004 19:30

John,

Also, there's liquid metal embrittlement, which occurs when the zinc metal penetrates the grain boundaries of the steel and causes stress raisers to form, which will result in fracture if the section is subjected to loads. This type of embrittlement is also related to cope cracking. This of course is where the blockout has been oxy cut, and the cut edges are thought to cause micro cracking along the coped section, allowing zinc to penetrate the cracks and initiate stress raisers in the stressed area. The cracks that I've seen are in the radius of the cope. Drilling the radius rather than torching it seems to alleviate the problem. There is an excellent article in the October '95 edition of Modern Steel Construction magazine that addresses cope cracking in structural steel after galvanizing