Corrosion Testing
Corrosion testing is the controlled evaluation of how a metal, coating, or assembled part degrades when exposed to moisture, salts, acids, humidity, polluted gases, or process chemicals. It involves measuring rust, pitting, weight loss, coating failure, or electrochemical response. In CNC machining and millwork production, it validates material choice, plating, anodizing, passivation, paint, sealants, packaging, and cutting-fluid compatibility before parts enter service.
On the shop floor, corrosion testing is built into first-article validation or supplier qualification for parts that face outdoor exposure, washdown, marine air, cutting fluids, or chemical contact. Common lab methods include salt spray testing in a chamber with 5% sodium chloride at 35°C, cyclic corrosion testing for wet/dry/humidity cycles, immersion testing for direct chemical exposure, and mixed flowing gas testing for polluted atmospheres. Surface preparation of machined coupons matters: edges should be smooth, cold-worked edges removed, and dissimilar-metal contamination avoided because grinding dust, chips, or rough sheared edges distort results. For CNC work, this is especially relevant for aluminum aircraft parts, stainless components, plated fasteners, and tooling or coolant-contact surfaces. Coupons are exposed to realistic environments, then inspected for pitting, rust, surface attack, or mass loss. Results are reported as time to initial rust, weight loss, thickness loss, time to perforation, or electrochemical changes, tied to service-life expectations.
- Bad coupon preparation: Machined specimens with burrs, cold-worked edges, grinding smear, or mixed-metal contamination can show premature attack that reflects fabrication damage rather than the true base material or coating.
- Wrong test for the service environment: Salt spray ranks coatings but may not predict outdoor cyclic weathering, washdown, or industrial gas exposure, so cyclic corrosion and mixed-flowing-gas tests are used when the real environment is more complex.
- Misreading accelerated failure as real-world failure: A part that performs poorly in a harsh lab chamber may still be acceptable in service, and one that survives a cabinet test may fail elsewhere; lab results must be correlated with field results.
What does ASTM B117-style salt spray actually measure?
It measures resistance to visible corrosion, rust, or pitting in a constant saline fog, not direct service-life in a real environment. It is useful for comparative coating evaluation but must be correlated with field conditions.
Why is surface roughness important in corrosion testing?
Rougher surfaces retain salts, moisture, and process residues more easily, which increases localized attack and can make coatings appear worse than they are on a smoother production finish.
Why are separate machines recommended for dissimilar metals during coupon prep?
Steel, brass, and aluminum chips can transfer between parts and create galvanic contamination or false corrosion sites, especially during grinding or abrasive finishing.