Corrosion Rate
Corrosion rate is the speed at which a metal deteriorates from chemical or electrochemical attack by its environment, commonly reported in mm/year, mils per year, or mass loss over time. In machining literature, it is derived from immersion or electrochemical test data, often inferred from corrosion current density. It reflects material, surface, and environmental conditions.
On the shop floor, corrosion rate enters material selection and process planning for components exposed to salt, humidity, coolants, or galvanic contact. Machining studies show surface condition and parameters alter corrosion behavior: 316 stainless in 3.5% sodium chloride is reported at 0.001 mm/year, while 304 sits near 0.01 mm/year. Corrosion rate is not simply a bulk property. Higher cutting speed with lower feed can reduce corrosion rate under some conditions, while lower roughness and better surface integrity correlate with lower corrosion current density. A rough, thermally damaged, or coolant-contaminated surface can become the first site for pitting or crevice corrosion. Fastener holes, inserts, edge-banded composite interfaces, and aluminum-steel joints trap moisture and dissimilar metals, accelerating attack. Salt spray, immersion, and electrochemical testing help estimate service life and coating performance. Treating corrosion rate as a process-dependent variable guides alloy choice, cleanup, and final assembly decisions.
How is corrosion rate measured in lab or production qualification?
Common methods include gravimetric mass-loss testing, salt spray exposure, and electrochemical techniques such as polarization and impedance; mass-loss testing can be converted to mm/year using density, exposed area, and time.
Why can two parts of the same alloy show different corrosion rates after machining?
Machining changes surface roughness, cold work, residual stress, and passive film behavior, so the near-surface condition—not just the bulk alloy—controls electrochemical response.
Which is better for corrosion resistance, 304 or 316 stainless in chloride exposure?
316 is better in chloride environments; one machining source reports 0.001 mm/year for 316 versus 0.01 mm/year for 304 in 3.5% sodium chloride solution.