Characteristics of Different Hot-Dip Galvanized and Zinc-Alloy Coatings

Formation Mechanism of Hot-Dip Galvanized Coatings

Hot-dip galvanization is a metallurgical reaction process characterized by two dynamic equilibriums: thermal equilibrium and zinc-iron exchange equilibrium. Microscopically, as steel articles are immersed in molten zinc at around 450°C, the articles absorb heat from the zinc, reaching temperatures above 200°C. At this point, the interaction between zinc and iron becomes apparent, with zinc infiltrating the surface of the iron article.

As the article’s temperature approaches that of the molten zinc, a layered structure with varying zinc-iron ratios forms on the article’s surface, constituting the stratified structure of the zinc coating. Over time, different alloy layers within the coating exhibit distinct growth rates. Macroscopically, the process involves the immersion of the article in the zinc, leading to boiling in the zinc bath. As the zinc-iron reaction gradually balances, the zinc bath becomes calm. When the article is removed from the zinc bath, and its temperature decreases to below 200°C, the zinc-iron reaction stops, and the hot-dip galvanized coating is formed, with its thickness determined.

Requirements for Hot-Dip Galvanized Coating Thickness

Several factors influence the thickness of the zinc coating, including the metal composition of the substrate, surface roughness of the steel, the presence and distribution of active elements such as silicon and phosphorus, internal stresses in the steel, geometric dimensions of the article, and the hot-dip galvanizing process.

International and Chinese hot-dip galvanizing standards currently categorize coating thickness based on steel thickness. The average and local thickness of the coating should meet specified requirements to ensure corrosion resistance. Different articles with varying thicknesses require different times to achieve thermal and zinc-iron exchange equilibrium, resulting in varying coating thicknesses. Coating thickness standards are based on industrial production experience considering the uneven distribution of coating thickness and the empirical values needed for corrosion resistance.

Therefore, ISO, ASTM, JIS, and Chinese standards have slightly different requirements for zinc coating thickness, but the differences are relatively minor.

Characteristics of Different Hot-Dip Galvanized Coatings

1. Hot-Dip Pure Zinc Coated Steel (GI Plate)

Currently, GI plates are produced with the addition of 0.2% Al in the zinc bath. The addition of Al improves zinc bath fluidity, and the reaction between aluminum and iron forms an iron-aluminum alloy layer, enhancing coating adhesion. The introduction of a small amount of aluminum in the zinc bath also forms an aluminum oxide film on the zinc surface, preventing surface oxidation and reducing zinc consumption.

GI plates, utilizing sacrificial anode (zinc) cathodic protection mechanisms, are widely used in various industries such as construction, appliances, automotive, transportation, and agriculture.

GI plate coatings exhibit a surface composed of a platform and uniformly distributed concave pits. The coating surface roughness is controlled to approximately 1μm through post-galvanization rolling. The GI plate consists of a top pure zinc layer, a steel substrate, and a thin inhibitory layer, known as the iron-aluminum alloy phase layer (FeAl3 or Fe2Al5). The iron-aluminum alloy phase prevents iron from diffusing into the zinc layer, avoiding the formation of relatively brittle zinc-iron alloy phases, ensuring coating adhesion.

2. Hot-Dip Zinc-Alloy Coated Steel (GA Plate)

GA plates undergo heat treatment at 500-550°C after galvanization, allowing iron and zinc in the steel substrate to mutually diffuse, forming a zinc-iron alloy phase layer. The coating’s surface iron content is approximately 10% (by mass).

GA plates require post-galvanization heat treatment to facilitate the diffusion of zinc and iron, forming a zinc-iron alloy phase. To reduce alloying time and the inhibitory layer’s blocking effect, the Al content in the zinc bath is generally lower than that in GI plates, around 0.13%. The formation of the zinc-iron alloy phase increases coating brittleness, making the coating prone to powdering or peeling during deformation, affecting the lifespan of forming molds.

The process of forming the zinc-iron alloy phase layer increases coating surface roughness, reducing color brilliance and darkening the coating‘s appearance. Greater roughness improves the coating’s paintability and enhances resistance to sand and stone impact. Therefore, compared to GI plates, GA plates exhibit superior corrosion resistance, weldability, paintability, and resistance to sand and stone impact. However, their forming performance is relatively poorer, and they lack the brightness of GI plates.

GA plates are commonly used in the automotive industry, primarily for Japanese and Korean vehicles. Stringent production control is required for the production of GA plates with zinc-iron alloy phase layers, necessitating precise process control. Domestic production capabilities for GA plates have matured.

GA plate coatings consist of rough zinc-iron alloy phases (δ1p and ξ phases), with the outermost layer being a combination of columnar crystalline δ1p and a denser δ1k layer close to the substrate. A 1μm thick Γ phase layer is present at the boundary between the coating and the substrate.

3. Hot-Dip Aluminum-Zinc Coated Steel (GL Plate)

Hot-dip aluminum-zinc coatings generally include Galfan (GF) coatings with 5% Al and Galvalume (GL) coatings with 55% Al. Continuous strip hot-dip aluminum-zinc coatings typically refer to Galvalume steel plates (GL plates) with 55.0% Al, 43.4% Zn, and 1.6% Si.

GL plates, with a higher aluminum content in the coating, exhibit corrosion resistance and high-temperature oxidation resistance due to aluminum. The presence of zinc provides cathodic protection. GL plates are widely used in construction, automotive, appliances, agriculture, and directly applied in applications such as mufflers, exhaust pipes, refrigerator backs, electronic microwave ovens, and heat exchangers.

The zinc flower diameter on the surface of GL coatings generally ranges from 1-3mm. GL coatings consist of two layers: an outer aluminum-zinc alloy layer comprising dendritic-rich aluminum solid solution and zinc-rich phases, and an inner layer of aluminum-zinc-iron intermetallic compounds. Similar to GI coatings, the inner aluminum-zinc-iron intermetallic layer in GL coatings prevents iron from entering the aluminum-zinc alloy layer, enhancing coating adhesion. The addition of silicon restricts the growth of brittle aluminum-zinc-iron intermetallic compounds.