Blog

What is hot-dip galvanizing?

Hot dip galvanizing can be said to be the most widely used anti-corrosion measure for steel worldwide.

In the previous post, we talked about "coating thickness". Today, let's continue this topic and talk about the color of the coating.

Friends in the engineering industry should have noticed that sometimes the surface of hot-dip galvanized parts is shiny, sometimes gray, and sometimes a combination of brightness and darkness - that is, mottled.

Why is there such a big difference in the appearance of the finished product with the same zinc ingot and the same process flow? This is actually related to the principle of hot-dip galvanizing.

Color is related to matter, and matter is determined by its composition. Therefore, before discussing the principle, let's first talk about the structure and composition of hot-dip galvanized layer.

Hot dip galvanized layer

The hot-dip galvanized layer is not simply a single zinc layer.

It is actually composed of four layers, from the outside to the inside: ④ - η phase, ③ - Zeta phase, ② - δ phase, and ① - γ phase, as shown in Figure 4.

Figure 4 Structure of hot-dip galvanized layer

The main difference between these 4 layers lies in their composition:

④ Layer 3 is a pure zinc layer with a Zn content of 100%. Layer 3 is a zinc iron alloy layer with a Zn/Fe content of approximately 94%/6%. Layer 2 is also a zinc iron alloy layer with a Zn/Fe content of approximately 90%/10%. Layer 1 is also a zinc iron alloy layer with a Zn/Fe content of approximately 75%/25%

We all know that zinc itself is a bright silver color, while iron is a dark gray color. Reflected on the hot-dip galvanized layer, it means that the higher the zinc content, the brighter the surface, and the higher the iron content, the darker the surface. So, the four layers of hot-dip galvanizing gradually deepen in color from the outside to the inside.

Principle of galvanized layer formation

The temperature of the zinc bath is generally between 455 ℃ and 480 ℃, and even high-temperature galvanizing will be controlled between 530 ℃ and 560 ℃.

Under such high temperature conditions, the iron atoms on the surface of the galvanized part after acid washing and plating assistance will diffuse into the zinc solution near its surface, combine with zinc to form a solid zinc iron alloy layer. The closer it is to the substrate surface, the more iron elements there are and the less zinc elements there are. Therefore, the closer to the substrate, the darker the color.

In addition to temperature, the chemical composition of the substrate also affects the color of the galvanized layer to a certain extent, with Si element having the greatest impact.

The editor wrote a niche post a long time ago, specifically introducing the influence of Si element on the thickness of coatings. Interested friends can refer to: Material Discussion | The Influence of Silicon (Si) Element in Base Metal on Hot Dip Galvanization

According to foreign research results, Si element not only affects the thickness of the coating, but also has a nonlinear effect on the surface color of hot-dip galvanizing (Sandelin effect), as shown in Figure 5.

Figure 5 Relationship between color, thickness and Si content of hot-dip galvanizing

It should be noted that the zinc pool temperature based on Figure 5 varies between 435-460 ℃, and the temperature curve changes accordingly. Figure 5 can be roughly divided into four stages:

<0.04%: Retain the η phase, color - bright silver/shiny, standard thickness, good adhesion

0.04% -0.15%: Zeta phase "eats" η phase, color - dark/matte, with color difference, very thick, decreased adhesion, increased brittleness

0.15% -0.25%: The alloy layer is normal, restored to brightness, but still slightly dark

＞ 0.25%: Increased again, dark gray

It should be noted that Si element often remains as a deoxidizer in steel during smelting, and it can catalyze the reaction between Fe and Zn at a certain temperature.

Different color causes

Let's talk about bright silver first

After the galvanized parts are taken out of the pot, if they are instantly cooled by water, the pure zinc liquid on the surface of the coating will cool down and solidify instantly, causing the iron atoms to be unable to continue diffusing and forming zinc iron alloy. Instead, an η phase - pure zinc layer will form on the outermost layer. This layer is the commonly seen bright silver color.

Besides, dark gray

On the contrary, if there is a certain window period after the galvanized part is taken out of the pot and before entering the water, the residual temperature of the galvanized part is still high enough to allow iron atoms to continue to diffuse, and the Zeta phase and δ phase continue to grow and "erode" the outermost η phase until the pure zinc layer is completely replaced by the zinc iron alloy layer, then the surface of the finished product will appear gray.

Finally, let's talk about mottled colors

The local color difference of the same galvanized part is mainly due to uneven surface cooling rate after the workpiece is taken out of the pot.

So, the factors that affect the cooling rate can be regarded as the causes of color difference. Especially when many galvanized components are stacked together, the heat dissipation rate of the core is slower, the temperature is higher, and the duration is longer than that of the periphery. Its η phase is more easily "sacrificed" and the color is darker.

In general, the color of hot-dip galvanizing is influenced by factors such as temperature, chemical composition, and galvanizing process. When there are special requirements for color, professional personnel need to communicate comprehensively with the galvanizing plant based on appearance and performance requirements. For example, whether color adjustment leads to process adjustment, how the adjusted process affects the mechanical properties of the workpiece and the adhesion of the zinc layer, and so on.

It is said that time is a god.

In fact, even if there is color difference, this phenomenon will gradually disappear after six months to two years, which means that surfaces with color difference will eventually converge in color.