February 13, 2025

Stainless steel passivation is a crucial surface treatment process in industrial manufacturing, especially in high-demand fields such as medical, food and marine engineering, which is directly related to the corrosion resistance, safety and service life of the product. Although traditional passivation methods, such as chemical pickling and electrolytic treatment, have certain effects, they have problems such as large pollution, high cost and strong limitations. Mechanical sandblasting passivation, represented by ceramic blasting beads, has become a new trend in stainless steel surface treatment due to its environmental protection, high efficiency and precise controllability.

This article will analyze in detail the basic principles of stainless steel passivation, compare common methods, and focus on how the ceramic beads play a key role in the passivation process.

Passivation refers to the formation of a dense chromium oxide film (Cr₂O₃) on the surface of stainless steel by physical or chemical means. This film is usually only 1-5 nanometers thick, but it plays an extremely important protective role (reference source: Journal of Materials Science & Technology, 2021)

1. Isolate corrosive media: prevent oxygen, moisture and chloride ions from penetrating, thereby slowing down electrochemical corrosion (reference source: Corrosion Science, 2020)
2. Improve surface stability: reduce active sites and reduce metal ion dissolution through lattice reconstruction.
3. Enhance wear resistance: The hardness of the oxide film can reach HV 800-1200, which is much higher than the exposed base metal (reference source: Surface and Coatings Technology)

The passivation process is strictly required in many industries, mainly to improve the corrosion resistance and cleanliness of stainless steel products. For example:

• Medical device industry: ASTM F86 stipulates that surgical instruments must pass a salt spray test of ≥500 hours after passivation to ensure that the surface is free of rust.
• Food industry: FDA 21 CFR 175.300 requires that food contact stainless steel surfaces must be passivated to prevent microbial growth and food contamination.
• Marine engineering: ISO 9227 standard stipulates that in a high-salt environment, the porosity of the passivation film on the stainless steel surface must be less than 1% to avoid pitting problems.

However, how to achieve high-quality passivation efficiently and environmentally friendly remains an important challenge facing the industry.

Currently, stainless steel passivation mainly uses three methods: chemical passivation, electrolytic passivation, and mechanical passivation (sandblasting), each of which has its own advantages and disadvantages.

Principle: Use nitric acid (20%-50%) or citric acid (4%-10%) solution to remove surface iron elements and form a chromium-rich oxide layer.

Advantages: low cost, suitable for workpieces with simple geometric shapes.

Disadvantages:

1. Wastewater contains heavy metals, and the treatment cost is high (reference source: Environmental Science & Technology)
2. It may cause micro cracks on the surface and reduce corrosion resistance (the corrosion rate of a certain automobile factory increased by 15% after pickling).

Principle: The workpiece is immersed in the electrolyte and energized to form a uniform oxide film.

Advantages: The film layer is uniform and controllable (1-10nm), suitable for high-precision electronic components.

Disadvantages:

1. High equipment investment (single equipment ≥ 50,000 US dollars).
2. High energy consumption (power consumption ≥ 20kW per hour, reference source: Journal of Materials Processing Technology)

Principle: Sandblasting removes pollutants, activates the surface, and promotes the natural oxidation of Cr₂O₃.

Advantages:

1. Zero chemical pollution, in line with ISO 14001 environmental protection standards.
2. Controllable surface roughness (Ra 0.2-2.5µm) enhances the oxide film’s adhesion.

Challenges:

Sandblasting parameters (pressure, angle, media selection) need to be precisely controlled.

The rolling effect of spherical particles: ceramic beads (sphericity ≥ 95%) peels off the oxide scale through rolling friction, avoiding cutting damage caused by angular media (such as aluminum oxide).

Precise particle size classification:

• B60 (60-100µm): cleaning of thick oxide scale and welding slag.
• B120 (20-40µm): removal of light stains and fingerprint residues.
• B205 (5-10µm): ultra-fine polishing.

Uniform pits (0.5-2µm) are formed on the surface after sandblasting, and the specific surface area increases by 30%, promoting oxidation reaction.

Lattice distortion effect: Sandblasting impact increases Cr diffusion rate and improves corrosion resistance (reference source: Acta Materialia)

Medical device industry

• Problem: Pickling causes intergranular corrosion of implants, with a scrap rate of 12%.
• Solution: B120 ceramic beads + citric acid passivation.
• Result: The scrap rate dropped to 0.5%, and the salt spray test pass rate reached 99.5%.

Food fermentation tank industry

• Problem: The residual of traditional sandblasting media causes excessive microorganisms.
• Solution: B60 ceramic beads + high-pressure water washing.
• Result: The colony was reduced by 92%, meeting FDA standards.

Summary:

Ceramic beads systematically solve the six pain points of traditional passivation through the three-step action of physical cleaning-surface activation-oxidation strengthening, and achieve a triangular balance of efficiency, cost and quality.

Sandblasting parameters:

Material hardness ≤HRC 30: B120 ceramic beads, PSI=80, incident angle 30°.

Material hardness＞HRC 30: B60 ceramic beads, PSI=120, incident angle 45°.

Quality inspection standard: White glove test has no contamination residue, and roughness meter test Ra meets the standard.

Chemical enhancement (optional):

• lNitric acid passivation: concentration 20%, temperature 25℃, soak for 20 minutes.
• lCitric acid passivation: concentration 10%, temperature 40℃, soak for 30 minutes.

Oxidation control: Real-time monitoring of ORP (oxidation-reduction potential) to maintain 200-300mV.

Copper salt test: According to ASTM A380 standard, the surface is qualified if there is no copper deposition.

XPS analysis: Cr₂O₃ accounts for ≥18% (untreated parts are usually ≤12%).

Ceramic blasting beads optimize stainless steel passivation through a three-step method of cleaning, activation, and strengthening, making it widely used in medical, food, marine and other industries. Compared with traditional methods, ceramic beads not only improves the quality of the passivation film, but also reduces environmental pollution and production costs, providing a more sustainable development path for the industry.