How can the spraying process be used to construct a corrosion protection coating system for valve pipelines?

Establishing a protective system for anti-corrosion coatings on valve pipelines is a systematic project that requires comprehensive consideration of multiple aspects such as materials, processes, environment, and quality control. As a core component, the standardization and reliability of the spraying process directly determine the performance and lifespan of the coating. The following are the detailed steps and key points for establishing this system:

## I. Preliminary Assessment and Design (Fundamentals)

1. **Operating condition analysis:**

    * **Medium type:** water, oil, gas, acid, alkali, salt, solvent, etc., as well as their concentration, temperature, and pressure.

    * **Operating temperature:** maximum, minimum, average operating temperature, and temperature variation range.

    * **Corrosive environment:** atmospheric environment (C1-C5, CX), soil environment, seawater environment, chemical plant environment, high temperature environment, etc.

    * **Mechanical stress:** vibration, impact, wear (particle erosion), stress state.

    * **Service life requirement:** Expected protection duration.

    **Regulatory standards:** Relevant national and industry standards (such as NACE, ISO, SSPC, GB, etc.) that must be adhered to.

2. **Confirmation of substrate:**

    * Valve/pipe material (carbon steel, stainless steel, alloy steel, cast iron, etc.).

    * Surface condition (newly manufactured, repaired, old coating condition).

    * Geometric complexity (valve body, flange, bolt, valve stem, internal chamber, etc.).

3. **Coating system design:**

    * **Selecting an anti-corrosion coating system:** Based on the analysis of working conditions, select the appropriate combination of primer, intermediate coat, and topcoat. Common systems include:

        * **Epoxy zinc-rich primer + Epoxy MIO intermediate coat + Polyurethane topcoat:** High versatility, excellent weather resistance, chemical resistance, and good mechanical properties.

        * **Inorganic zinc-rich primer + epoxy intermediate coat + fluorocarbon/polysiloxane topcoat:** High temperature resistance, long-term corrosion resistance, and super weatherability.

        * **Phenolic epoxy coating:** Resistant to high temperatures, strong solvents, and chemical erosion (suitable for harsh environments).

        * **Glass flake coating:** Excellent penetration resistance and wear resistance (suitable for severe corrosion or wear environments).

        * **Polyurea:** fast curing, impact resistance, wear resistance, seamless (suitable for areas requiring rapid construction or high wear resistance).

    * **Determining Dry Film Thickness (DFT):** Determine the DFT of each coating layer and the total DFT based on the corrosive environment, expected lifespan, and coating characteristics.

    * **Clear compatibility:** Ensure good compatibility and adhesion between the primer, intermediate coat, and topcoat.

## II. Surface treatment (key)

This is the most crucial link in the success or failure of the entire anti-corrosion system! **

1. **Cleaning:**

    * Thoroughly remove oil, grease, dust, salt, soluble salt contaminants (such as chloride ions, sulfate ions), etc. Commonly used solvents for cleaning, alkaline cleaners, or specialized degreasing agents.

    * Testing standard: **Soluble salt content** (as per ISO 8502-6, 8502-9) must meet the standard (typically requiring <20 µg/cm² NaCl equivalent, with stricter requirements for harsh environments).

2. **Rust removal:**

    * **Sandblasting and cleaning:**

        * **Selection of abrasives:** Steel shot, steel grit, angular sand, copper slag, garnet, etc. Selection is based on the hardness of the substrate, the original rust grade, the required roughness, and environmental protection requirements.

        * **Cleanliness level:** It must reach **ISO 8501-1 Sa 2.5 (near-white level) or Sa 3 (white level)**. This is the minimum requirement for achieving good coating adhesion. For critical parts or harsh environments, Sa 3 is the preferred choice.

        * **Roughness:** Create an appropriate anchor pattern. Typically within the range of **50-100 micrometers (2-4 mils)**, it needs to match the selected coating (usually 1/3 to 1/2 of the total coating thickness). Commonly measured using a comparison plate (such as ISO 8503-1 Rugotest No. 3) or a profilometer.

    * **Power tool cleaning:** Only applicable to areas where sandblasting is not feasible or for small-scale repairs (such as St 3 level). The effect is far inferior to sandblasting, and careful selection of paint and design thickness is required.

    * **Acid pickling/chemical conversion:** Primarily used for specific metals (such as stainless steel acid pickling and passivation), it is not commonly used as the primary rust removal method in the corrosion prevention of carbon steel valve pipelines.

3. **Protection after surface treatment:**

    * After sandblasting, the surface becomes highly reactive and requires the application of primer within **the specified timeframe (usually within 4-8 hours, or as per the paint instructions)** to prevent flash rusting and secondary contamination. In high humidity or adverse weather conditions, the time window is even shorter.

## III. Spraying Construction Process (Core)

1. **Environmental control:**

    **Temperature and humidity:** It must be within the range specified in the paint product manual (usually, the substrate temperature should be at least 3°C higher than the dew point temperature, the ambient temperature should be between 5-40°C, and the relative humidity should be less than 85%. Some paints have even lower requirements). Monitor with a temperature and humidity meter.

    * **Ventilation:** Ensure good ventilation to exhaust solvent vapor and dust, thereby guaranteeing construction safety (explosion prevention) and coating quality (reducing pinholes and solvent entrapment).

    * **Cleanliness:** The construction area should be as dust-free, oil-free, and splash-free as possible. Open-air construction is prohibited during rainy, snowy, or windy weather.

2. **Preparation of paint:**

    **Mixing:** Strictly follow the proportions specified in the product manual, and use a dedicated mixing device to thoroughly mix two-component or multi-component coatings until they are fully homogenized.

    * **Induction:** Some coatings need to be left undisturbed for a certain period of time (Induction Time) after mixing to allow the reaction to fully proceed.

    * **Dilution:** Only use the specified diluent to adjust the viscosity to suit the spraying equipment within the allowable range specified in the instructions. Over-dilution can seriously affect the performance of the coating.

    * **Filtration:** The mixed paint needs to be filtered through a screen of appropriate mesh size to remove impurities and gel particles.

3. **Spraying equipment and parameters:**

    * **Equipment selection:**

        * **Airless spraying:** It is the most commonly used method, with high efficiency and a wide range of applicable viscosities, capable of achieving thicker coatings. It is the primary spraying method for corrosion protection of valve pipelines. **

        * **Air spraying:** Good atomization effect, good coating appearance, suitable for topcoats or complex parts, but low material utilization rate, prone to overspray, relatively difficult to control film thickness.

        * **Mixed air spraying:** Combines the advantages of airless and air spraying.

        * **Electrostatic spraying:** It improves material utilization and is suitable for pipelines with regular shapes, but the complex structure of valves may affect the shielding effect.

        * **Special parts:** For hard-to-reach areas such as inside the valve cavity and bolts, small spray guns, internal hole spray guns, or brush/roller coating can be used for supplementation.

    * **Parameter setting:**

        * **Pressure:** Set appropriate pump pressure and atomization pressure based on equipment type, paint viscosity, and nozzle model.

        * **Nozzle selection:** Select the correct nozzle model (aperture, angle) based on the required flow rate, atomization effect, and film thickness.

        * **Spray distance:** Maintain a suitable and constant distance between the spray gun and the workpiece surface (typically 30-40cm for airless spraying).

        * **Moving speed:** A uniform and steady moving speed is the key to achieving a uniform film thickness.

        * **Lapping:** Ensure sufficient overlap (usually 50%) between spray patterns to avoid missed spraying or excessively low film thickness.

4. **Spraying operation:**

    * **Follow the "Wet on Wet" principle:** Apply the next coat within the specified overcoating interval of the primer (usually from the time it is touch dry to before the maximum overcoating interval), to achieve the best interlayer adhesion. If the maximum overcoating interval is exceeded, roughening treatment is required.

    * **Handling of complex parts:**

        * **Flange face:** The inner edges of bolt holes and the edges of the flange sealing surface are key protection areas that require precise operation to avoid coating contamination of the sealing surface (usually, the sealing surface needs to be covered or scraped off after coating).

        * **Bolts and nuts:** It is recommended to use specialized anti-corrosion paste or strippable paint, or apply them carefully.

        * **Valve cavity:** Ensure that the spray gun can reach into and cover all internal surfaces, paying special attention to the flow passage and valve seat areas. A specialized spray bar or rotary spraying may be required.

        * **Weld seams, edges, and corners:** These areas are prone to insufficient film thickness, and should be pre-coated (carefully brushed or sprayed with a small spray gun) before being sprayed as a whole.

    * **Film thickness control:**

        * **Wet film thickness measurement:** During the construction process, use a wet film thickness gauge for real-time monitoring to guide the adjustment of spraying parameters.

        * **Dry film thickness measurement:** After each coat has cured, use a magnetic or eddy current dry film thickness gauge to conduct multi-point measurements (following the 90-10 or 80-20 rule). **Ensure that the design-required DFT is achieved, and avoid excessive thickness (which may lead to cracking, solvent entrapment, and cost waste).**. **

## IV. Curing and Maintenance

1. **Curing conditions:** Strictly follow the requirements of the paint instructions to provide the necessary temperature, humidity, and time for curing. Low temperature or high humidity can significantly prolong the curing time, and may even lead to wet paint or reduced performance of the coating.

2. **Protection:** Avoid rain, condensation, mechanical damage, trampling, and contamination until the coating is fully cured to a strength sufficient for handling, installation, or exposure to the medium.

3. **Post-curing:** Some high-performance coatings (such as phenolic epoxy) may require post-curing at elevated temperatures to achieve optimal performance.

## V. Quality Inspection and Acceptance

1. **Process inspection:**

    * Surface treatment cleanliness (visual inspection, soluble salt test), roughness.

    * Environmental parameters (temperature, humidity, dew point).

    * Wet film thickness.

    * Coating appearance (presence or absence of defects such as sagging, missed spots, pinholes, blistering, and orange peel).

2. **Final inspection:**

    * **Dry film thickness:** Conduct comprehensive inspection to ensure compliance with design requirements.

    * **Coating appearance:** uniform and continuous, with no severe defects.

    * **Adhesion Test:** Conduct cross-cut test (ISO 2409/ASTM D3359) or pull-off test (ISO 4624/ASTM D4541) on key areas to ensure compliance with the specified grade.

    * **Pinhole detection:** For key anti-corrosion areas (such as buried pipelines and the inner walls of storage tanks), use a low-pressure wet sponge or a high-pressure electric spark leak detector to test the continuity of the coating, ensuring there are no pinholes.

    * **Missed spot detection:** Use when necessary.

    * **Curing degree detection:** It is determined by solvent wiping method (MEK test) or hardness test (Barcol hardness tester).

3. **Acceptance Criteria:** Acceptance shall be conducted in accordance with the contract, technical specifications, and relevant standards (such as NACE SP0188, ISO 21809, GB/T 30790, etc.). A comprehensive inspection report shall be generated.

## VI. Special Precautions (for Valve Pipes)

* **Inside the valve:** Ensure that the internal cavity, valve seat, valve plate/ball/gate plate, and all surfaces that come into contact with the flowing and contacting media are effectively protected. Spraying is difficult and requires special attention.

* **Movable parts:** Moving components such as valve stems and valve stem nuts require coatings with good flexibility and wear resistance to prevent cracking and peeling due to movement. Sometimes, special lubrication and sealing designs are necessary.

* **Flange sealing surface:** **Under no circumstances should paint be applied!** It must be strictly protected using specialized masking tape or removable protective agent. Before installation, bolt holes and sealing surfaces must be thoroughly cleaned.

* **Thread:** It is recommended to use thread anti-rust grease or removable thread locking sealant to prevent thread seizure caused by ordinary paint.

* **Marking and nameplate:** They must be clearly visible after spray painting.

## Summary

To build a successful anti-corrosion coating protection system for valve pipelines, **surface treatment is the cornerstone, coating design is the direction, spraying process is the core, and quality control is the guarantee**. Each link must be strictly controlled, from early evaluation, material selection, surface treatment, environmental control, standardized construction to strict inspection, to ensure that the coating can effectively protect equipment for a long time in the expected corrosive environment, reduce maintenance costs, and ensure production safety and environmental safety. For complex valves, more effort and special techniques/tools are required for the treatment of their internal and special parts.

**Remember: Without proper surface treatment (Sa 2.5/Sa 3 + low salt content + suitable roughness), even the best paint and spraying techniques cannot achieve the desired anti-corrosion effect. **