Maximum Coating Thickness
Is it possible to have too much galvanized coating on a steel part?
The specifications for hot-dip galvanizing do not set a maximum coating thickness, so there is no limit on the thickness of the hot-dip galvanized coating. However, there is a requirement the coating meets the intended use of the product, so if there are drips and runs in an area that will cause a fit problem in the parts final assembly, then the drips and runs must be ground smooth for the part to be acceptable.
Although the specification does not limit the coating thickness, there are some practical limits on how thick the galvanized coating should be. When the coating exceeds 10 mils or 250 microns, the coating experiences significant stress during the cooldown from galvanizing temperatures. The thermal contraction of steel progresses at a different rate than the thermal contraction of the intermetallic layers of the galvanized coating. This difference in contraction rates induces a stress at the interface between layers of the coating. The stress level induced depends on the thickness of the intermetallic layers. At normal coating thickness levels, the stress is minimal and there is no issue with the coating adherence. However, if the coating is thick then the stress can be significant, and this makes the coating susceptible to flaking as seen in the photo below.
The stress at the interface between layers may not cause immediate flaking of the coating but assembly, shipping or any other type of additional stress on the coating can cause a separation between the intermetallic layers and a flaking of the coating. Any time the coating is above the 10 mil or 250-micron thickness this flaking is a potential. The situation is more serious when the steel part is thick. Sheet metal parts cool very quickly so they dont usually experience this type of flaking but thick beams and poles can be very susceptible to flaking, of this type. All of this means it is very important to know when steel will have a very thick coating by knowing the steel chemistry or have a good idea from past jobs that the particular steel or part will have thick coatings and will be susceptible to potential flaking.
Besides the cost issues with adding 2 to 8 times as much zinc as is needed to the galvanized coating, the issue of flaking can make the part unacceptable to the customer and the added cost of stripping and re-galvanizing can hurt the bottom line. If the flaking does not show up until the part is at a job site, the shipping and other costs can turn this situation into a real nightmare. If the coating looks too thick, then measure the thickness so you know if there is a potential of flaking. If it is very thick, then test for flaking before you send the part to the job site. These steps will help save some of the potential extra costs for a flaking situation.
© 2024 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.
Comments
Dessie Melese
we have a contract to install tower members. But the requirements are 215micro of galvanization thickness. Is there any quality problem in this regards. the area has been reported hostile and more chemical reactions.
(AGA)
Hi Dessie, Yes there could be issues with this request. We cover these concerns in the following article: https://galvanizeit.org/knowledgebase/article/maximum-coating-thickness
Adnaan Malak / falcon zinc Metal industry
One of my client has been asking me to restrict the coating thickness limit Of galavanizing to 227 microns . For a thickness of steel greater than 10mm with silicon content of 0.15- 0.35 %. His concern his that there is a possible chance of flaking beyond 227 microns . But is that true for the above specs and if yes . What kind of flaking test can be conducted on the steel to ensure its adhesion
(AGA)
Hi Adnaan, Limiting the coating thickness is not required, and depending on the part it may or may not be achievable to limit the coating thickness. Yes, flaking can be a potential concern for coatings greater than 200-250 microns. To limit coating thickness, blast cleaning before galvanizing can be effective for reactive steels where immersion times cannot be limited. An adhesion test procedure is provided in ASTM A123 specification.
Adnaan Malak / falcon zinc Metal industry
My client also insists on carrying out bend test for high coating thickness sample plates to check the adhesion capability of the zinc with base steel . Is this appropriate?
Adnaan Malak / falcon zinc Metal industry
Also if a part consists of a base plate of 80mm with a column welded of 30mm to it . You tend to have a higher coating thickness on the 30mm part . In such cases , is the higher coating thickness development of more than 250 microns subjected to approval or not ?
(AGA)
Hi Adnaan, No, bend test is a test for embrittlement and not adhesion. The test for adhesion is achieved using a stout knife and smoothly running it along the surface of the steel without whittling and gouging. The procedure for this test is found in ASTM A123/A123M . The knife should put a slight mark in the zinc metal surface, but should not cause any flaking or delamination of the layers. Paring or whittling with the knife is not acceptable, and the test should not be performed on corners or edges of the product. Coating thickness requirement is a minimum, and there is no maximum coating thickness requirement. For more info: https://galvanizeit.org/knowledgebase/article/maximum-coating-thickness
Falcon Zinc Metal Industry
I have galvanized fabricated structural steels materials of EN 10025-2 S355 JR grade and the thickness of different parts vary from 6mm to 40mm. At some locations Galvanizing thickness observed about 400 microns. I have conducted adhesion test -Stout Knife test as per ASTM A 123 on those area with high coating thickness and found no peeling off during the test. Kindly confirm if the materials are acceptable or do i need to conduct any additional test to check for flaking. Materials galvanized as per ASTM A 123 and ISO 1461. After reading the article of "Maximum Coating thickness" of AGA , my client is afraid that there will be potential flaking of zinc coating on the galvanized materials.
(AGA)
Hi Falcon, If the parts meet ASTM A123 requirements and the stout knife test in A123 was requested and passed, the parts are acceptable. Just because a coating is thicker than 8-10 mils doesn't mean the coating will flake -- it just means there can be an increased potential for flaking. For example, we sometimes see tightly adherent coatings near 20 mils. Care should be taken during handling and erection by the customer (including the use of washers beneath any turning pieces during any fastener installations).
Pallab Dutta
Hi, we have column with base plate 150mm, flange 90mm, web 75mm and other stubs plate with minimum 50mm thickness. Thickness of galvanizing coming on average of 1000 microns and upto 1500 microns and above also. Flaking is happening during knife test or hammer test, but it can remove with little more pressure by chisel. Is it acceptable? What should galvanizer do to minimize the coating thickness? Chemical composition, including SI values are ideal for galvanizing.
(AGA)
Knife test according to ASTM A123 should be used to test for adherence and acceptability. To minimize coating thickness, parts can be blast cleaned before galvanizing or the galvanizer can attempt to limit immersion time in the galvanizing kettle, among other methods. Predicting steel reactivity using chemical composition report is not an exact science. Element levels can vary +/- 0.02% and the values listed on the mill test report are only one sample taken from the heat. The element levels on the individual pieces, and even on the same piece of steel, can vary to some degree. Additionally, be aware that some foreign steel producers are known to have inaccurate steel chemistry analyses on the mill reports.