Sampling Standard: ASTM B602 defining statistical attribute sampling procedures for metallic coating inspection and quality acceptance.

Summary

ASTM B602 is the statistical sampling standard for inspecting plated parts rather than a coating specification itself. It defines how many parts you inspect from production lots and what defect levels are acceptable for pass/fail decisions. Think of it as the rulebook for statistically valid quality control. The spec prevents inspecting every single part (impossible for high volume) while catching bad lots reliably. Based on military standard MIL-STD-105 acceptance sampling tables adapted for plating industry needs.

Coatings

B602 applies to all electroplated and inorganic coatings regardless of metal type. Statistical sampling methodology works universally across coating processes and materials.

• Zinc
• Nickel
• Gold

Requirements Overview

B602 defines inspection levels and acceptable quality limits (AQL) creating statistical sampling plans. Inspection level determines sample size relative to lot size. AQL sets maximum acceptable defect percentage balancing quality requirements with inspection costs. Normal, tightened, and reduced inspection modes adjust scrutiny based on supplier history. The standard provides lookup tables showing how many parts to inspect and maximum allowable defects before rejecting entire lot.

Sampling Plan Selection: Choosing inspection levels and AQL values balancing quality confidence with inspection costs for production lot acceptance.

Requirements Selection

Level II inspection is industry standard unless application demands tighter or proven supplier allows relaxed control. AQL selection depends on defect consequence. Critical characteristics affecting safety or function need AQL 0.25 or tighter. Major defects impacting performance use AQL 1.0 to 1.5. Minor cosmetic defects accept AQL 2.5 to 4.0. Start with normal inspection then adjust based on supplier performance. Multiple AQL values often specified simultaneously for different defect severities on same parts.

Underplate Layer Stacks

B602 doesn't specify coating systems but sampling plans apply to verifying underplate presence and quality. Typical inspection checks include nickel underplate thickness before topcoat, adhesion between layers, and proper sequence of plating steps. Destructive testing of sample parts from lot validates complete coating system meets specifications. Statistical sampling prevents inspecting every layer on every part while catching process deviations reliably.

• Thickness Verification: Sampling for coating build

  B602 sampling determines how many parts measured for thickness compliance. XRF or other nondestructive methods check topcoat. Destructive sectioning on sample parts verifies underplate thickness and layer sequence meeting coating system specifications.

• Adhesion Testing: Sample-based quality checks

  Statistical sampling defines how many parts subjected to adhesion testing like bend tests or tape tests. Complete coating system validated through destructive testing of representative samples rather than testing every production part.

• Visual Inspection: Sampling for defects

  B602 tables determine sample size for visual examination detecting coating defects like skips, blisters, or discoloration. Sample inspection catches process upsets affecting appearance or coverage without 100% screening entire production lot.

Sampling Errors: Common mistakes in applying statistical inspection causing inadequate quality control or excessive inspection costs.

Common Mistakes

Biggest mistake is misunderstanding what AQL means. AQL 1.0 doesn't mean 1% defects are acceptable. It's statistical limit where 95% of lots at that defect level will pass sampling plan. Actual defect rates run lower. Using same AQL for critical and minor defects wastes inspection resources or under-inspects important characteristics. Failing to switch to tightened inspection after repeated failures lets bad supplier continue. Switching to reduced inspection prematurely before process proven stable invites quality escapes.

Conclusion

ASTM B602 provides statistically valid sampling methodology for plating inspection avoiding expensive 100% screening while catching bad lots reliably. The standard's beauty is mathematical rigor preventing arguments about sample sizes and accept/reject criteria. Proper application requires understanding AQL concept, selecting appropriate inspection levels, and following severity switching rules. Used correctly, B602 optimizes inspection costs while maintaining quality confidence. It's the foundation for receiving inspection at plating shops and incoming inspection at customer facilities for all types of electroplated and inorganic coatings.