Q1: What are the common anti-corrosion methods for ordinary carbon steel pipes?
The main anti-corrosion methods for ordinary carbon steel pipes include hot-dip galvanizing, epoxy coating, paint protection and cathodic protection. Hot-dip galvanizing is to immerse the steel pipe in 450℃ molten zinc liquid to form an alloy coating. The thickness of the zinc layer is usually 50-80μm, which can provide 10-50 years of protection. Epoxy powder coating forms a dense protective film through electrostatic spraying and high-temperature curing, which is suitable for buried pipelines. Paint protection has low cost but poor durability and requires regular maintenance. Cathodic protection is often used in conjunction with coating to inhibit corrosion through sacrificial anodes or impressed currents. The selection of anti-corrosion solutions requires comprehensive consideration of environmental corrosiveness, service life and cost budget.
Q2: What are the process characteristics of hot-dip galvanized carbon steel pipes?
The production of hot-dip galvanized carbon steel pipes requires key processes such as degreasing, pickling, plating, drying and zinc dipping. Pickling uses hydrochloric acid or sulfuric acid to remove the surface oxide scale, and the plating aid (usually zinc ammonium chloride solution) can prevent rusting and promote the zinc-iron reaction. The zinc immersion temperature is controlled at 445-465℃ for 3-5 minutes to form a composite structure of zinc-iron alloy layer (Γ phase) and pure zinc layer (η phase). After plating, air knife blowing is required to control the thickness of the zinc layer, and water cooling is performed to set the shape. This process can achieve metallurgical bonding between the coating and the substrate, and the adhesion is significantly better than electroplating zinc.
Q3: What are the key points of epoxy resin coating construction?
Before epoxy resin coating, sandblasting must be performed to ensure that the surface cleanliness reaches Sa2.5 level and the roughness is 40-80μm. Two-component epoxy coatings must be strictly mixed in proportion and applied after aging for 20 minutes. The ambient temperature during spraying should be higher than 5℃ and the relative humidity should be lower than 85% to avoid condensation affecting adhesion. The coating is usually divided into three layers: primer, intermediate paint and topcoat, with a total dry film thickness of ≥200μm. Ventilation is required during curing, and full performance can only be achieved after 7 days. This coating has excellent chemical corrosion resistance and wear resistance, but it is easy to powder under ultraviolet radiation, and polyurethane topcoat is required for outdoor use.
Q4: Anti-corrosion strategy for ordinary carbon steel pipes in marine environments?
Carbon steel pipes in marine environments face multiple threats such as chloride ion corrosion, wave scouring and microbial attachment. Typical protection schemes are: three-layer PE anti-corrosion system (epoxy primer + adhesive + polyethylene jacket), or glass flake epoxy coating, dry film thickness ≥ 500μm. 316L stainless steel coating or monel alloy sheath is used for key parts. The design should avoid water accumulation structure and increase sacrificial anode (such as zinc block) protection. Regular underwater inspection and coating repair, if necessary, cathodic protection and coating combined protection can be used, and the design life can reach more than 20 years.
Q5: How to detect the quality of the anti-corrosion layer of carbon steel pipes?
The anti-corrosion layer inspection includes three stages: before construction, during construction, and after construction: before construction, the quality of the substrate surface treatment is inspected (such as the cross-cut method to measure adhesion); during construction, a wet film thickness gauge is used to control the coating thickness, and a spark leak detector (test voltage 5kV/mm) is used to detect pinholes; after curing, a magnetic thickness gauge is used to measure the dry film thickness, and the pull-off method is used to test adhesion (≥5MPa). Buried pipelines also need to undergo cathodic protection potential testing (-850mV to -1.2V vs CSE). In addition, samples can be taken for accelerated aging tests such as salt spray tests (such as ISO 9227) and chemical medium resistance immersion tests to evaluate long-term performance.
