How to improve the corrosion resistance of stainless steel cast pipes through surface treatment

As a key industrial fluid conveying component, the service life and stability of cast stainless steel pipes depend to a great extent on their corrosion resistance. In the fields of chemical, pharmaceutical, marine engineering, etc., the working environment is often full of corrosive media, such as chlorides, sulfides, organic acids, inorganic acids, etc. In order to improve the stable performance of cast stainless steel pipes in complex corrosive environments, the surface treatment process has become an important factor in determining the corrosion resistance of the product.

The significance and role of surface treatment
During the casting process, stainless steel cast pipes may have defects such as scale, sand holes, slag inclusions, and microcracks on the surface. These surface defects not only affect the appearance quality, but also become potential causes of corrosion. Scientific surface treatment processes can effectively remove the surface contamination layer, form a dense passivation film, and improve the ability to resist pitting, intergranular corrosion, crevice corrosion, etc.

Pickling treatment: removal of scale and impurities
Pickling is the most common primary surface treatment method for cast stainless steel pipes, often using a mixture of nitric acid and hydrofluoric acid. Its functions are as follows:
Remove the black oxide scale and residual inclusions on the surface formed during the casting process
Remove the oxide film and metal contamination layer generated after heat treatment
Provide a clean substrate surface for subsequent passivation treatment
The surface shows a uniform silver-white color after pickling, which helps to further improve the surface activity of the material and facilitate the formation of a complete passivation film layer.

Passivation treatment: Building a stable protective film
Passivation treatment generates a chromium-rich oxide film on the surface of stainless steel through chemical action. This film is dense and stable and is a key barrier to prevent corrosion. Commonly used passivation liquids are oxidizing media such as nitric acid or phosphoric acid.
The passivated stainless steel cast pipe has the following performance advantages:
Significantly improve the resistance to intergranular corrosion
Inhibit the occurrence of surface pitting and crevice corrosion
Improve surface stability and extend service life
The passivation film formed after passivation treatment is generally 1~5 nanometers thick. Although it is thin, it is extremely dense and has excellent self-healing properties. It can recover quickly after damage.

Electrolytic polishing: Improve finish and corrosion resistance
Electrolytic polishing uses electrochemical action to preferentially dissolve the microscopic protrusions on the surface of stainless steel, thereby achieving a flat, smooth, and mirror-like surface. Compared with mechanical polishing, electrolytic polishing has no mechanical stress residue and is more suitable for the internal and external surface treatment of cast pipes with complex geometric shapes.
The advantages are as follows:
Significantly improve surface finish (Ra value can be as low as 0.2μm)
Reduce bacterial adhesion and sediment accumulation, and improve cleanliness
Effectively improve the uniformity and stability of the surface passivation film
Improve resistance to chloride ion corrosion, suitable for the food and pharmaceutical industries
The stainless steel surface is denser after electrolytic polishing, effectively blocking the penetration of corrosive media and extending the service life of cast pipes in highly corrosive environments.

Mechanical sandblasting and shot blasting: uniform cleaning and surface activation
Sandblasting uses high-pressure air to spray quartz sand, ceramic sand, etc. onto the surface of the cast pipe to remove surface oxide scale, sand holes and sticky sand to form a uniform rough surface. It is suitable for pretreatment before pickling and can also be used as an activation process before coating treatment.
Advantages include:
Evenly clean the inner and outer surfaces of complex shapes
Eliminate surface defects during casting
Improve the efficiency of subsequent pickling and passivation treatments
Provide a good adhesion foundation for spraying or coating
The surface cleanliness of the cast pipe treated by sandblasting is high, which is conducive to the formation of a higher quality passivation film or anti-corrosion coating.

Coating protection (optional): Enhance protection against extreme environments
For stainless steel cast pipes used in offshore platforms, salt spray areas or sour gas environments, coating protection can be added to the surface. Common surface coatings include epoxy coatings, PTFE anti-corrosion layers, ceramic composite coatings, etc.
The advantages of coating treatment are:
Provide additional physical isolation to prevent corrosive media from directly contacting the metal
Improve surface wear resistance and impact resistance
Extend the service life in extreme environments
Choose the appropriate coating type and thickness, which can be customized according to specific working conditions to improve the overall performance of the product.

Inspection and quality control after surface treatment
After the surface treatment of stainless steel cast pipes is completed, strict quality inspection is required. Common methods include:
Passivation film integrity test (iron ion test, ferric chloride pitting test)
Surface roughness test (using a roughness meter to evaluate the Ra value)
Surface cleanliness inspection (visual inspection, optical magnification)
Salt spray test and corrosion simulation test (to verify the treatment effect)
Ensure that the surface treatment of each stainless steel cast pipe meets industry standards and ensure the reliability and safety of customers during use.