Argentum Solutions Inc. - Sterling Guidance on Corrosion and Materials Degradation

Argentum Solutions, Inc.

Sterling guidance on corrosion and materials degradation

Potential-pH Diagrams

THERMEXPERT - Potential-pH diagram generator

Intelligent Tools

POLEXPERT - Polarization Scan Artificial Neural Network Expert System

SEQEXPERT - Sequential Immersion Test Artificial Neural Network Expert System

CYLEXPERT - Rotating Cylinder Electrode Intelligent Rotation Rate Calculator

Corrosion Calculator

Corrosion Economics Estimator

FINCALCULATOR - Corrosion Economic Calculator

TUTORIAL ON POLARIZATION RESISTANCE TECHNIQUE (Misnamed LINEAR POLARIZATION)

David C. Silverman

Table of Contents

Overview of Tutorial
Summary of Polarization Resistance Technique
Sources of Error

Example (Nickel in Strong Acid)
Example (Low Corrosion Rate Environment)

Overview of Tutorial

Predicting rates of general corrosion by electrochemical techniques is far easier than predicting the risk of localized attack such as crevice corrosion or pitting. The linear polarization technique or as it should be more properly called, the polarization resistance technique is one such technique for rapid estimates of general corrosion. The name "linear polarization" technique is actually a misnomer because it implies linearity at the corrosion potential. The inverse relationship between the polarization resistance (slope of the voltage versus current curve at the corrosion potential that this technique can estimate) and the corrosion current (corrosion rate) exists even though the curve itself is often not linear at the corrosion potential.

The methodology for generating the potentiodynamic sweep in the vicinity of the corrosion potential is adequately described in a number of references, one excellent example is ASTM G59 ”Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements”. This standard test method developed using a laboratory round robin testing program provides a test circuit and a standard test protocol (430ss in sulfuric acid) useful for determining if the electrochemical equipment is functioning properly. The polarization resistance technique is very well established for routine use both for corrosion prediction and corrosion monitoring. During routine use, the realistic lower limit for corrosion rate estimation is about 0.001 mm/y (about 0.1 mpy) mostly because of limitations in estimating Tafel slopes. Under quiescent conditions and with no outside electrical or other interference, a maximum polarization resistance of about 10⁶ ohm-cm² has been suggested. Such values would translate to corrosion rates of about 10^-4 mm/y or about 0.01 mpy. Sensors are available for on-line corrosion monitoring from several commercial suppliers.

This tutorial has several objectives:

to provide a brief overview of the technique and data analysis
to summarize the assumptions behind estimation of the corrosion rate from the current vs. voltage (polarization resistance) plot
to summarize a number of the sources of error
to provide a simple example in the use of the technique
to provide an example in a very low corrosion rate environment that can cause offsets to the corrosion potential

Next Page: Polarization Resistance Technique

Return to Table of Contents

David C. Silverman, Ph.D. - Primary Consultant
E-Mail:     dcsilverman@argentumsolutions.com
Phone:     314-576-3586
Fax:         314-754-9825
Address:   The Argentum House
                14314 Strawbridge Ct.
                Chesterfield, MO 63017