Q1: What are the commonly used anti-corrosion methods for Q235 steel pipe?
Hot-dip galvanizing (zinc layer ≥ 80μm, GB/T13912) is suitable for outdoor structures and can last up to 20 years. Epoxy coal tar coating (three oils and two cloths) is used for buried pipes, which is acid and alkali resistant and low cost. Plastic spraying (epoxy powder) thickness is 60~120μm, suitable for dry indoor environment. Cold galvanizing (electroplating) process is simple but has poor protection (zinc layer ≤ 20μm). Aluminum-magnesium alloy spraying (Arc Spray) can be used in special environments, with a temperature resistance of 500℃.
Q2: How to detect the quality of the galvanized layer of Q235 steel pipe?
The thickness of the zinc layer is measured by a magnetic thickness gauge (GB/T4956), and the double-sided average value is ≥85g/m² (≈12μm). The copper sulfate test (GB/T2694) is qualified if it is soaked 4 times without exposing iron. Adhesion test uses the cross-cut method (1mm spacing), and the peeling area after the tape is torn off is ≤5%. The appearance inspection must be free of defects such as sagging and missing plating. After galvanizing, passivation treatment (chromate or chromium-free passivation) is required to enhance corrosion resistance.
Q3: What are the protective measures for Q235 steel pipes in marine environments?
It is recommended to use a three-layer system of "epoxy zinc-rich primer (80μm) + epoxy micaceous iron intermediate paint (100μm) + polyurethane topcoat (60μm)". The tidal range area can be covered with glass fiber reinforced plastic (FRP) or monel alloy sheath. Sacrificial anode protection (aluminum alloy anode) must be designed and installed according to ISO15589. Underwater potential detection (-0.85~-1.1V vs CSE) is carried out regularly (every 2 years). Duplex stainless steel composite pipes should be used instead of pure Q235 for key parts.
Q4: How to treat the inner wall of Q235 steel pipe for corrosion protection?
Drinking water pipelines are lined with cement mortar (GB/T50268) with a thickness of 4~10mm. IPN8710 non-toxic coating (dry film ≥200μm) is commonly used for oil pipelines. The food industry can electropolish stainless steel lined composite pipes with a degree of Ra0.8μm. When transporting chemical media, the lining plastic (PE/PP) material must be selected according to the pH value. Before internal anti-corrosion construction, sandblasting must be carried out to Sa2.5 level (GB/T8923), and the anchor pattern depth is 40~70μm.
Q5: How to repair corroded Q235 steel pipes?
Local corrosion (pit depth ≤10% wall thickness) can be repaired by arc spraying (Zn/Al), and it needs to be polished to St3 level before spraying. Large-area corrosion requires replacement of pipe sections, and the new and old pipes are connected with a V-shaped groove full penetration structure. The repair of the outer anti-corrosion layer of the buried pipe requires excavation of 1.5 times the pipe diameter, and layered re-coating according to the original standard. After repair, a 48-hour pressure test (1.5 times the working pressure) is required. Severely corroded (remaining wall thickness < 70% of the design value) must be replaced as a whole.
