The chemical properties of metal bridges (usually made of steel, aluminium alloy or stainless steel) depend largely on the material they are made of, but steel galvanised bridges are generally the most common. The following is a detailed analysis of their chemical properties:

1. Main materials and chemical properties

(1) Galvanised steel bridge (most common)

Base material: low carbon steel (Fe-based, containing a small amount of C, Mn, etc.).

Coating: Surface hot dip galvanised (Zn), thickness usually 45~100μm to provide cathodic protection.

Chemical properties of zinc:

Forms a dense ZnO/ZnCO₃ protective film in air, preventing further corrosion.

Dissolves in acidic or alkaline environments:

Zn+2HCl→ZnCl2+H2↑(reacts with acid)Zn+2HCl→ZnCl2+H2↑(reacts with acid)Zn+2NaOH+2H2O→Na2[Zn(OH)4]+H2↑(reacts with strong base)Zn+2NaOH+2H2 O→Na2[Zn(OH)4]+H2↑(react with strong base)

(2) Stainless steel bridge (corrosion-resistant type)

Material: 304 (06Cr19Ni10) or 316 (06Cr17Ni12Mo2) stainless steel.

Characteristics:

Passivation film (Cr₂O₃) is formed by Cr to resist corrosion, resistant to acid, alkali and salt, but pitting corrosion may occur in Cl-containing (e.g. seawater) environments.

(3) Aluminium alloy bridge (lightweight)

Material: 6063 or 5052 aluminium alloy (Al-Mg-Si system).

Characteristics:

Natural oxide film (Al₂O₃) on the surface is resistant to atmospheric corrosion, but not resistant to strong acids and alkalis:

2Al+6HCl→2AlCl3+3H2↑2Al+6HCl→2AlCl3+3H2↑

2. Comparison of corrosion resistance

Material Acid Resistance Alkali Resistance Salt Mist Resistance Applicable Environment

Galvanised steel Weak Medium Strong Indoor dry environment, light industrial environment 

Stainless steel Strong Strong Extremely strong Chemical, coastal, high humidity environments 

Aluminium alloy Weak Weak Medium Dry indoor, non-acidic and alkaline corrosive environments 

3. Common chemical reactions and corrosion mechanisms

(1) Corrosion of galvanised layers

White rust: The galvanised layer reacts with O₂ and CO₂ in a humid environment to produce Zn(OH)₂-ZnCO₃ (white loose material), which needs to be cleaned up in time.

Red rust: If the galvanised layer is broken, the steel substrate will rust when exposed (Fe₂O₃-nH₂O).

(2) Passivation and pitting of stainless steel

Passivation: Cr and O₂ form a dense Cr₂O₃ film, preventing further reaction.

Pitting: Cl- destroys the passivation film, triggering localised corrosion (e.g. 316 stainless steel for coastal areas).

(3) galvanic coupling corrosion of aluminium alloy

When in contact with dissimilar metals such as copper and steel, the corrosion of aluminium is accelerated due to potential difference (insulation treatment is required).

4. Protective measures

Galvanised bridge:

Avoid contact with acids and alkalis (e.g. chemical plants) and check the integrity of the plating regularly.

Damage can be repaired by spraying zinc paint (cold galvanising).

Stainless steel bridges:

Clean regularly with neutral detergent to prevent Cl- accumulation.

Aluminium alloy bridge:

Avoid direct contact with copper and steel, insulating gasket can be added.

5. Environmental impact

High temperature and humidity: accelerate the oxidation of galvanised layer and corrosion of steel substrate.

Industrial pollution: SO₂, H₂S and other gases will promote acid rain corrosion.

Summarise

Galvanised steel bridge: cost-effective, suitable for general environment, but need to prevent acid and alkali.

Stainless steel bridge: strong corrosion resistance, suitable for harsh environments, higher cost.

Aluminium alloy bridge: lightweight and beautiful, but lower mechanical strength and corrosion resistance.