Provide a detailed introduction to the surface treatment process of sanitary-grade stainless steel

The surface treatment process of sanitary-grade stainless steel pipes is the core technology that ensures their high cleanliness, corrosion resistance and anti-contamination capabilities. The design of this process directly affects the applicability of the pipes in industries such as pharmaceuticals, healthcare, and food. The following provides a detailed analysis from the dimensions of process classification, treatment process, technical characteristics, and quality standards: 

I. Core Objectives and Classification of Surface Treatment Processes

1. Core Objectives

Reduce surface roughness (Ra value): Minimize residue of media and attachment sites for microorganisms. Ideally, Ra ≤ 0.2 μm.

Form a passivation film: Enhance corrosion resistance and prevent the release of metal ions (such as Fe³⁺).

Eliminate surface defects: Such as weld spurs, scratches, oxide scales, etc., to avoid accumulation of dirt and contaminants.

2. Main Process Classification

II. Detailed Explanation of Core Surface Treatment Processes

1. Mechanical Polishing (MP)

Processing Flow

Coarse Polishing: Use a grinding wheel (80-120 mesh) to remove welding splashes and oxide films, and initially level the surface.

Medium Polishing: Use a sanding belt (180-320 mesh) for further grinding, eliminating the traces of coarse polishing, and reducing the Ra value to below 1.6 μm.

Fine Polishing: Use a wool wheel + polishing paste (600-1000 mesh) for precise grinding, with Ra reaching 0.8 μm.

Technical Characteristics

Advantages: Low equipment cost, suitable for batch processing of non-critical weld areas; can correct macroscopic defects on the pipe surface.

Limitations: It is a physical polishing process, which may cause surface hardening (crystal lattice distortion), and is difficult to handle complex geometries (such as the inner side of a bend); polishing paste residues need to be thoroughly cleaned; otherwise, organic contamination may be introduced.

Application Scenarios

Non-sterile filling lines in the food and beverage industry, ordinary purified water pipelines (with a cleanliness requirement of ≤ Ra 0.8 μm).

2. Electro Polishing (EP)

Processing Principle

Utilizing electrochemical dissolution principle: Use the pipe as the anode, immerse it in an electrolyte (such as a mixture of phosphoric acid + sulfuric acid + chromic acid), and through current, the microscopic protrusions on the surface are preferentially dissolved to achieve a leveling effect.

Processing Flow

1. Pre-treatment: Degreasing (alkali cleaning) → Acid washing (nitric acid + hydrofluoric acid to remove oxide films).

2. Electro Polishing:

Voltage: 8-20V, Temperature: 50-80℃, Time: 5-15 minutes.

Electrolyte composition control: Phosphoric acid accounts for 60-70%, determining the polishing rate; sulfuric acid regulates viscosity, and chromic acid enhances the brightness.

3. Post-treatment: Deionized water rinse → Neutralization (sodium carbonate solution) → Drying.

Technical Parameters and Advantages

Surface Effect: Ra can reach 0.2-0.5 μm, forming a uniform passivation film (Cr₂O₃ content is 30% higher than that of mechanical polishing).

Unique Advantages:

No mechanical stress: Avoids surface hardening caused by traditional polishing, suitable for thin-walled pipes (wall thickness ≤ 2mm).

Good Uniformity: The current distribution is uniform, and the inner walls of complex parts such as bends and trisockets can also be polished.

Self-cleaning effect: Dissolved products are discharged in the form of bubbles, reducing impurity residues.

Application Cases

An EP-treated 316L stainless steel pipe used in an mRNA vaccine production line showed that the surface microbial attachment was reduced by more than 90% compared to the mechanically polished pipe.

3. Passivation Treatment

Process Essence

By dissolving free iron ions on the surface with chemical reagents (such as 60-70% nitric acid solution), the stainless steel surface forms a dense Cr₂O₃ passivation film (thickness approximately 2-3 nm), enhancing corrosion resistance.

Processing Flow (using nitric acid passivation as an example)

1. Pre-cleaning: Deionized water rinse to remove surface oil, metal debris.

Passivation Treatment:

Temperature: Room temperature - 50℃, Time: 20-60 minutes.

Concentration Control: Insufficient nitric acid concentration will result in incomplete passivation film, while too high a concentration may cause corrosion of the base material.

3. Neutralization and Detection: Use ammonia water to adjust pH to neutral, and confirm the passivation effect through blue dot test (detecting free iron).

Key Function

Repairing oxidized film: The passivation film damaged during mechanical polishing or welding can be regenerated through passivation treatment.

Reducing metal ion release: In a pH=3 acidic solution, the Fe ion release from 316L pipes is less than 0.1 ppm after passivation.

4. Electrochemical Polishing (ECP)

Differences from Electro Polishing Electrolyte optimization: Add organic amine additives (such as ethylenediamine) to inhibit hydrogen gas release and improve polishing accuracy.

More stringent process parameters: Voltage control accuracy ±0.5V, temperature fluctuation ≤2℃, time accurate to the second level.

Ultra-perfect surface effect

Ra ≤ 0.2μm, surface reflection rate over 85%, approaching mirror-like effect; suitable for pipelines in contact with highly active drugs (such as anti-tumor drugs), avoiding drug adsorption and deterioration. 

III. Quality Inspection and Standards after Surface Treatment

1. Roughness Inspection (Ra Value)

Inspection Tools: Portable Roughness Meter (e.g., Mitutoyo SJ-210), measure 3-5 points along the axial direction of the pipe material, the average value should comply with industry standards (such as FDA requires that the Ra of pipes in direct contact with drugs should be ≤ 0.5 μm).

2. Corrosion Resistance Test

Salt Spray Test: 5% NaCl solution, 35℃ constant temperature spray for 240 hours, no rust spots on the surface (ISO 9227 standard).

Intergranular Corrosion Test: According to ASTM A262 standard, immerse in boiling sulfuric acid - copper sulfate solution for 16 hours, no cracks after bending 180°.

3. Cleanliness Verification

TOC (Total Organic Carbon) Inspection: Residue of polishing paste ≤ 0.5 ppm (pharmaceutical industry standard).

Microbial Test: Cotton swab wipe the surface, after cultivation, the number of colonies ≤ 10 CFU/100 cm². 

IV. Combination of Different Processes and Industry Selection Logic

1. Basic Combination: Mechanical Polishing + Passivation

2. Standard Combination: Electrolytic Polishing + Passivation+ Passivation + Coating

V. Frontier Trends in Surface Treatment Processes

1. Chrome-free Electrolytic Polishing: Utilizes a sulfuric acid - phosphoric acid - organic carboxylic acid system instead of chromic electrolyte, complying with environmental regulations (such as the EU REACH regulations).

2. Plasma Polishing: Utilizes plasma bombardment on the surface (temperature ≤ 100℃), suitable for heat-sensitive medical devices (such as endoscope tubes).

3. Intelligent Monitoring System: Through real-time monitoring of parameters such as electrolyte conductivity and temperature, automatically adjusts the polishing process to ensure consistency of the surface of each batch of tubes. 

Summary

The surface treatment process of sanitary-grade stainless steel pipes is the core part of "cleanliness control": mechanical polishing addresses the problem of macroscopic leveling, electrolytic/electrochemical polishing achieves microscopic precise leveling, and passivation treatment enhances corrosion resistance at the material level. The combination of different processes needs to be selected comprehensively based on industry standards (such as FDA, GMP), medium characteristics (such as the acidity and alkalinity of the liquid), and cost budget. The ultimate goal is to block the contamination path through surface optimization and ensure product safety and process stability.