Potentiodynamic
What does the potentiodynamic curve actually show?
It shows how current changes as the electrode potential is swept, revealing active dissolution, passive film formation, and possible breakdown events.
Why is scan rate important?
The measured response depends on how fast the potential is swept; faster or slower sweeps can shift apparent corrosion or breakdown features and affect interpretation.
What is the most relevant output for shop QA?
Usually Ecorr and Icorr, because they are commonly used to compare surface condition, coating quality, and corrosion susceptibility between process variants.
Potentiodynamic testing is an electrochemical method in which a potentiostat sweeps the potential of a metal electrode at a controlled rate while recording the resulting current. It is used to evaluate corrosion behavior, passivation, and pitting resistance, producing parameters such as corrosion potential (Ecorr) and corrosion current density (Icorr) per standards like ASTM G59.
In a CNC or QC setting, potentiodynamic testing is used to verify how machining affects corrosion performance. Stainless steel brackets, aluminum housings, plated fasteners, and welded assemblies that see coolant, salt, humidity, or cleaning chemicals are prime candidates. A potentiostat sweeps the part's potential in an electrolyte while a reference and counter electrode complete the circuit, and the resulting current response indicates whether the surface remains passive or breaks down into active dissolution or pitting. This makes the test valuable for comparing surface conditions: tool marks, heat tint, smeared metal, embedded free iron, grinding residue, or poor passivation show up as electrochemical differences even when dimensions are inside tolerance. The practical takeaway is that surface chemistry, not just geometry, determines corrosion resistance. For shops producing medical components, marine hardware, or parts headed for passivation or anodizing, the test provides comparative data to qualify cleaning and finishing processes.
Low Icorr mistaken for field durability: A favorable lab curve does not guarantee service performance when chloride level, temperature, aeration, or crevice geometry differ; the test is comparative, not a lifetime predictor.
Contaminated part blamed on alloy: Leftover free iron, swarf, or grinding residue changes the polarization response, so the tested surface no longer represents the base metal; passivation exists to strip away that contamination.
Confusing electrochemistry with metrology: Potentiodynamic testing measures electrochemical response, not dimensional change, so it cannot verify hole size, flatness, or thread quality; those still require calibrated inspection tools.