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What kind of coating can I expect when galvanizing an unfamiliar steel grade?

Most iron-containing materials are suitable for galvanizing, but how do you know what to expect when galvanizing an unfamiliar steel grade? Evaluation of the steel chemistry is critical in understanding if and how the material will galvanize. When this evaluation is performed by the galvanizer and the engineer/specifier together, steel chemistry can either be optimized in advance when ordering from the mill or adjustments can be made in the galvanizing process with the intent of meeting everyones expectations regarding final coating appearance and thickness.

Evaluating Steel Chemistry

To evaluate the chemistry for an unfamiliar steel grade, the elemental composition of the steel in weight percentage should be estimated from the mill test reports for the heat. Where mill reports (or accurate mill reports) cannot be obtained, it is possible to locate a range for each element in the steel by looking up Chemical Requirements tables within the steel grade specification or by acquiring ranges from the steel manufacturers website/brochure. The elements listed in ASTM A385 should be compared according to the recommendations in order to achieve a coating which is of typical appearance and thickness.


ElementRecommended % for HDGNotes
Si< 0.04 % or 0.15% - 0.22%Sandelin steels and steels high in Si content may produce thick, matte, and/or rough coatings
P< 0.04%P > 0.04% produce rough, thick coatings susceptible to delamination
Si Equivalent< 0.04 % or 0.15% - 0.22%Si Equivalent 0.04% - 0.15% or > 0.22% may produce thick, matte, and/or rough coatings
C< 0.25%Check ultimate tensile strength for steels >1% C
Mn< 1.3%High Mn may produce brownish colored and brittle coatings
Sandelin curve and effect of silicon on coating thicness
Sandelin Curve

The Silicon Equivalent

Steels with silicon and phosphorus levels which are beyond the recommended levels are considered reactive steels. Reactive steels are galvanized regularly, and the silicon equivalent is used to evaluate the combined effect silicon and phosphorus have on the metallurgical reaction during hot-dip galvanizing. After the Silicon equivalent has been calculated, look up the value on the Sandelin Curve to predict the reactivity of the steel.

Silicon Equivalent = Si content + 2.5*(P content)

  • Silicon equivalent <0.04% or 0.15% - 0.22% - coatings are of typical thickness and appearance.
  • Silicon equivalent 0.04% - 0.15% (Sandelin steels) - vary in appearance and develop thicker coatings. If a nickel alloyed zinc bath is used, the Sandelin effect is mitigated, resulting in a higher likelihood of bright coatings of typical coating thickness.
  • Silicon equivalent > 0.22% - matte gray and rough coatings of a thickness greater than the minimum requirements. See Sandelin curve to determine the level of reactivity and anticipated coating thickness.

Stainless Steels

Stainless steels of the 300 series can be galvanized because they contain nickel, which is necessary to initiate the reaction between the steel and the zinc. Alternatively, stainless steels of the 400 series do not contain nickel and cannot be galvanized successfully.

Weathering Steels

Weathering steels (ASTM A588, A709 Weathering, COR-TEN) typically develop thicker galvanized coatings due to higher levels of silicon (up to 0.40% allowed, and 0.27% to 0.35% common). Additionally, due to the initial surface roughness, coating weights are comparable for both pickled and blast-cleaned surfaces. Expect a matte gray coating with little or no spangle.

Low Silicon / Aluminum-killed Steels

Steels with very low levels of silicon (less than 0.02%) or aluminum-killed steels regularly present a challenge in developing a coating that meets the thickness requirements of ASTM A123 and A153.

Copper-Containing Steels

Some steel chemistries with small amounts of copper such as weathering steels can be successfully hot-dip galvanized. Pure copper cannot be galvanized. Coatings which are thicker and darker in appearance are typical.

High-Speed Machining Steels

Steels containing high amounts of sulfur (S >0.18%) are unsuitable for hot-dip galvanizing and will erode.

Other Considerations

There are additional considerations when evaluating steel chemistry, as predicting steel reactivity 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. However, using mill reports which state the estimated elemental composition typically leads to the most accurate evaluation. Additionally, be aware that some foreign steel producers are known to have inaccurate steel chemistry analyses on the mill reports. When in doubt, galvanize a test sample for confirmation.

Once the steel chemistry has been evaluated, be sure to also evaluate ultimate tensile strength, initial surface condition, the use of old or recycled steel, surface roughening, steel thickness, and the presence of any thermally cut edges to determine whether any of these conditions may have additional effects on the final coating thickness and appearance.

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Hargrove Engineers + Constructors

Can AISI 1045, 4140 or 4340 steels be successfully and reliably hot-dip galvanized?


Hello HEC, They can be galvanized, but there are some precautions. If these steels have been heat treated, carbon content can be high enough that there is little-to-no iron left remaining on the surface of the steel to react with the molten zinc in the galvanizing kettle (therefore unable to form the galvanized coating). Fastener manufacturers are aware of this issue with the potential for high carbon content at the surface and can mitigate this issue during the manufacturing process. Where we have seen issues with these steels in the past is when the material is purchased from the manufacturer, sent separately for heat treatment, and then sent separately to a galvanizer. Furthermore, we tend to see issues when the heat of the galvanizing bath (825 – 850 F) is not considered when galvanizing anchor bolts or other hardware tempered at temperatures near 800F. Hence, it is recommended to galvanize test pieces per each heat and test to confirm the mechanical properties are as intended.

Schmitz, Ed / WEG Electric Equipments

Hello Alana. My company produces galvanized radiators that will be used in power transformers. We buy the raw material from the mill, manufacture the radiators and send to a galvanizer. We are having some reactivity problems. First question: If we cannot reach Si Equiv <0.04% what could the galvanizer do in the galvanizing process to avoid high thickness and rough appearance? Second question: The steel mill claims that AGA considers Si Equiv values ​​up to 0,08 to 0,09% acceptable. That's true? Is there a specific Sandelin curve for Si Equiv?

Falcon Zinc Metal Industry

Hi Alana, My question the sandelien curve on the x-axis shows silicon equivalent or silicon %. if i have an MTC of a article with Si % of 0.14. Can i then plot the same on he curve and read the values based on the latter. or should i calculate the Si equivalent as you have shown above . Kindly clarify ?


Hello Falcon Zinc, When making an assessment with MTRs, calculate the Si equivalent% using the formula. Second, on the Sandelin curve you will plot both the Si% of 0.14 and the Si equivalent value you calculated. If both are in the reactive range, then you likely have reactive steel. If one says it is reactive and the other falls in the recommended range -- then the steel may or may not be reactive and only galvanizing a test piece will confirm.

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