Full Process & Test Specification Of Passivation For M0.6-M3 Stainless Micro Fasteners, Based On Current National Standard GB/T5267.4 & GB/T3098.6

Micro stainless steel screws are widely used in Bluetooth earphones, mobile phone camera modules and portable medical testing equipment. Ultra-small inner thread diameter and tight stacking make conventional stainless steel passivation process cause liquid residue, micro-hole blockage and foggy yellow surface, failing warehouse salt spray acceptance requirement of 48h~96h NSS for medical and 3C clients. All process parameters, test thresholds and defect judgment in this article are extracted from original national standard without fictional self-defined indicators, strictly distinguish passivation system matching 201, 304, 316 austenitic stainless steel grades, cover full-link control of incoming pre-treatment, passivation reaction, multi-stage pure water cleaning, constant temperature drying and finished sampling inspection. Combined with mass production defect statistics of over 100,000 pcs in precision electroplating workshop, objectively sort out real inducement and general industry rectification measures of four types of frequent defects.

1. Core Parameter Comparison Table Of Passivation Process For Three Common Stainless Steel Grades (Allowable Range Specified By GB/T5267.4 Standard)

2. Four Standardized Full Passivation Process Steps Mandated By National Standard

1. Pre-degreasing & Pickling Procedure (Mandatory Pre-process, Chapter 5 GB/T5267.4): Load micro screws into hollow PP roller, two-stage ultrasonic alkaline degreasing at 50℃ for 3 minutes to remove drawing oil and release agent left from cold heading forming. After clean water flushing, soak in 5% dilute nitric acid pickling tank for 1.5 minutes at ambient temperature to remove surface oxide scale and iron impurities. Conduct three-stage overflow pure water rinsing after pickling to ensure complete hydrophilic surface without water break film; oil residue will directly lead to local missing passivation film. 2. Chemical Passivation Reaction Procedure: Match corresponding passivation liquid according to material grade, full temperature control within table range, roller roll slowly at 4r/min to avoid tight stacking blocking liquid penetration into ultra-fine thread. Over-soaking over 20min for 316 grade will cause surface matte discoloration, damage appearance and salt spray performance. 3. Multi-stage Closed Pure Water Rinsing: After passivation, soak in three-stage countercurrent pure water for 2 minutes per level, resistivity of final stage pure water ≥10MΩ·cm to thoroughly wash residual passivation agent trapped in thread gap. Residual liquid crystal will form yellow spots and corrosion source after drying, the most common defect of micro fasteners. 4. Closed Constant Temperature Drying: Transfer rinsed workpieces to hot air drying tunnel, continuous drying at 70~80℃ for 15 minutes, keep slight roller rotation to prevent screw stacking adhesion. Drying temp below 60℃ leaves water vapor residue, over 90℃ cracks passivation film, both shorten salt spray durability cycle.

3. Objective Causes & Practical Rectification Plan For Four Mass Production Frequent Defects After Passivation

1. Large-area foggy yellow surface: Root cause is substandard final-stage pure water quality and aging invalid passivation tank liquid. Rectification: Test pure water resistivity daily, replace filter core once below standard; titrate passivation liquid concentration every 7 days, supplement main salt or replace whole tank, stabilize drying temp at 75℃. 2. Micro thread hole blockage & liquid crystal adhesion: Excessive single batch feeding, tight stacking inside roller, insufficient liquid circulation flow rate. Rectification: Reduce single feeding quantity by 40%, add hollow diversion partition inside roller, equip bottom circulation pump for continuous stirring, add high-pressure pure water spray to flush inner thread before drying. 3. White rust spot appears in advance during salt spray test: Iron impurities not fully removed in pickling process, uneven passivation film thickness. Rectification: Scoop iron precipitate from pickling tank bottom daily, extend ultrasonic degreasing time, sample cross-section film thickness test for same batch, control passivation film thickness between 0.1~0.3μm. 4. Uneven light & dark luster of finished goods: Unstable roller rotation speed, different contact conductive points of workpieces against barrel wall. Rectification: Fix roller low speed at 4r/min, mix same-size PP isolation beads inside barrel to disperse contact points, separate passivation by specification, forbid mixed processing of M0.8 and M3 screws.

4. NSS Salt Spray Sampling Judgment Rule Mandated Before Delivery (GB/T 10125)

Adopt AQL1.0 sampling standard for batch ≤50,000 pcs, extract 80pcs split into two groups: 40pcs for full appearance inspection, 40pcs for continuous neutral salt spray test. Whole batch unqualified if 304 grade fails to reach 48h rust-free or 316 grade fails 96h rust-free, all goods must be stripped and re-passivated, forbidden to screen qualified parts for direct delivery. Micro screws matched with medical equipment add damp heat cycle aging test per GB/T2423 standard before shipment.