Anti-Corrosion Surface Treatment Process of Stainless Steel Fasteners by Zhjiali, Passivation & Electropolishing for Better Salt Spray Resistance

Anti-Corrosion Surface Treatment Process of Stainless Steel Fasteners by Zhjiali, Passivation & Electropolishing for Better Salt Spray Resistance

Stainless steel relies on native passive film for rust prevention, but oil stains, iron scraps and tiny scratches during processing will damage the protective layer, leading to pitting corrosion and discoloration under coastal salt spray, acid and alkali vapor. Zhjiali Hardware specializes in stainless steel fastener manufacturing and builds standardized surface treatment workshop. Graded anti-corrosion solutions for SUS304 and SUS316 screws, bolts and micro torx screws greatly extend service life under outdoor conditions.

1. Two Main Anti-Corrosion Finishing Processes for Stainless Steel

1. Stainless Steel Passivation (General Standard Process)

Passivation is the basic anti-rust procedure for stainless steel. After degreasing to remove cold heading oil and metal debris, workpieces are soaked in special passivation solution to form dense chromium-rich oxide film. Passivated SUS304 fasteners reach over 1000 hours salt spray resistance, while SUS316 can exceed 2000 hours. The process does not change original dimension or gloss, and will not interfere with thread assembly. Widely applied to PV bracket bolts, outdoor equipment screws and general chemical fittings with balanced cost and anti-corrosion performance.

2. Electropolishing (For High-End Clean Applications)

Electropolishing micro-levels surface burrs and scratches via electrochemical reaction, while forming uniform passive film. Workpieces gain bright smooth surface without gaps to trap salt or bacteria, with superior corrosion resistance and high cleanliness. Higher processing cost, mainly used for medical devices, food machinery and micro electronic screws requiring strict surface finish standards.

2. Complete Standard Treatment Flow

1. Degreasing: High-temperature alkaline tank removes lubricants from cold heading and thread rolling; 2. Multi-stage pure water rinsing: Three times washing to avoid chemical residue and surface stains; 3. Acid neutralization: Remove free iron contamination to prevent later yellow rust; 4. Core passivation / electropolishing: Process selected according to working conditions, strictly control liquid concentration, soaking time and temperature; 5. Final rinse & hot air drying: Eliminate water stains before packaging; 6. Random salt spray test: Batches pass neutral salt spray inspection before delivery.

3. Corrosion Performance Difference Between Stainless Steel Grades

1. SUS304: Ideal for inland and slightly humid environments after passivation, not suitable for long-term coastal high chloride conditions; 2. SUS316: Molybdenum alloy greatly improves chloride resistance, matched with passivation for tidal PV, seawater equipment and chemical workshops; 3. Non-magnetic SUS316: Low-magnetism smelting plus electropolishing for medical testers and RF instruments without magnetic interference.

4. Quality Control During Processing

Independent processing area for stainless steel to avoid iron contamination from carbon steel parts. Separate tanks for different materials and processes to prevent cross-contamination. All liquid replacement cycles, temperature and soaking time are recorded, salt spray test reports archived for each batch. Low-concentration solution for tiny micro torx screws to avoid thread damage.

5. Targeted Solutions For Various Industries

Outdoor PV Projects: SUS316 bolts with passivation for coastal power stations to reduce maintenance frequency; Precision Medical Devices: Non-magnetic micro screws with electropolishing for clean medical assembly standards; Chemical & Water Treatment Equipment: Thickened passive film to resist long-term weak acid and alkali erosion; Indoor Electronic Equipment: Cost-effective passivated SUS304 screws for indoor anti-rust demands.

6. Stable Stock & Custom Machining Support

The factory holds full stock of SUS304 / SUS316 hex bolts, socket cap screws and tamper-proof torx screws. Custom anti-corrosion treatment available based on actual working environment. Non-standard shaped stainless steel fasteners can be processed with passivation or electropolishing, fast sample delivery and stable mass production scheduling.