English
## I. Basic Background
Hot-dip galvanized structural steel contains silicon, manganese, sulfur, phosphorus, oxygen, nitrogen, hydrogen and various intentionally added alloying elements in addition to carbon. Silicon and manganese are essential deoxidizing elements for smelting, while sulfur, phosphorus and gaseous elements originate from raw materials and smelting residues. These elements alter the binary iron-zinc reaction system, affect the growth rate of alloy phases, coating thickness and surface quality, and may even form a double-layer coating structure at high concentrations.
## II. Specific Effects of Each Element
### 1. Carbon
(1) Content effect: Increased carbon content intensifies iron-zinc reactions and accelerates the growth of iron-zinc alloy layers, leading to an overall thicker coating. Coating growth of low-carbon steel is diffusion-controlled. High-carbon steel preferentially promotes the growth of ζ phase and inhibits δ phase. When the mass fraction of carbon reaches 0.5%, the δ phase barely grows, and the coating is mainly composed of ζ phase.
(2) Existing form effect: Its influence is far greater than the carbon content alone:
- Cementite: It features high stability and weak reactivity with zinc. When distributed in pearlite in spherical or lamellar forms, it increases surface irregularities of the substrate, generates microcracks, accelerates iron atom diffusion and promotes iron-zinc reactions. When present as coarse particles or dispersed particles in martensite, it hardly affects iron-zinc reactions.
- Graphitic carbon: Graphite areas on the steel surface cannot react with zinc, directly resulting in bare spots.
(3) Summary of rules: Higher carbon content and larger pearlite fraction lead to thicker coatings; uniform distribution of carbides enables steady coating growth.
### 2. Silicon (Most Significant Influence, corresponding to the Sandelin effect)
(1) Core function: Silicon greatly increases the dissolution rate of iron in molten zinc and strengthens iron-zinc reactions. The coating thickness presents a special fluctuating curve with silicon content:
- w(Si) ＜ 0.03%: Slightly improved reaction activity with normal coating structure;
- 0.06% ~ 0.1%: Violent iron-zinc reactions and maximum coating thickness;
- w(Si) ≈ 0.18%: Reactions are suppressed and the coating becomes thinner;
- w(Si) ＞ 0.3%: Reaction rate rises again.
(2) Coating defects: High silicon content causes rapid growth of ζ phase extending towards the outer coating surface, resulting in rough, dull coatings with poor adhesion. Industrially, galvanizing process parameters are adjusted according to silicon content ranges, and standards specifying combined limits of silicon and phosphorus contents are formulated for galvanized steel.
### 3. Manganese and Sulfur
Low contents of both elements in ordinary low-carbon steel exert negligible influence on coating structure. For high-manganese steel during galvanizing, the proportion of ζ phase rises significantly, and Γ, δ, ζ and η phases can all form inside the coating.
### 4. Phosphorus
(1) Independent effect: Trace phosphorus accelerates the coarsening growth of ζ phase and inhibits δ phase, thickening the coating. Excessive growth of ζ phase squeezes the outer η pure zinc layer, dulling the coating surface and generating pockmarks. The promoting effect of phosphorus on iron-zinc reactions is approximately 2.5 times that of silicon.
(2) Silicon-phosphorus synergistic effect: The coexistence of the two elements drastically aggravates excessive coating thickness. Based on this phenomenon, component control standards for steel have been established in the industry: w(Si)+2.5w(P) ≤ 0.09% (or 0.11%) to avoid defective coatings.
### 5. Various Alloying Elements
- Manganese: When the mass fraction of manganese in steel exceeds 1.3%, it accelerates coating growth and facilitates ζ phase formation.
- Titanium, vanadium, niobium: Their inherent contents in steel have little impact on galvanizing. However, when aluminum is added to molten zinc, these elements destroy the Fe-Al barrier layer and accelerate iron-zinc reactions.
- Chromium, nickel: Steel containing over 11% chromium and over 5% nickel shows linearly increasing coating thickness with dipping time.
- Molybdenum: Low molybdenum content promotes iron-zinc reactions, while the element turns to suppress reactions when its mass fraction exceeds 0.5%.
### 6. Gaseous Elements (Nitrogen, Oxygen, Hydrogen)
- Nitrogen: It barely affects iron-zinc reactions when its mass fraction is within 0.02%.
- Oxygen: Existing in the form of oxide inclusions tends to induce excessively thick coatings.
- Hydrogen: Hydrogen penetrates into steel during pickling and precipitates under heating in galvanizing, generating internal pressure that causes cracking of alloy layers and accelerates iron-zinc corrosion reactions.
## Overall Process Conclusions
The contents of silicon, phosphorus, carbon and carbide morphology shall be controlled at the steel material selection stage. For special steels such as high-silicon steel and high-manganese steel, galvanizing temperature, dipping duration and aluminum content in molten zinc need to be adjusted to counteract coating defects including excessive thickness, rough surface, high brittleness and bare spots induced by harmful elements.