A
s concrete structures begin to age and deteriorate, the need to understand corrosion behavior is pertinent to determining where a structure is in its life cycle. One of the most successful methods of determining the corrosion rate of embedded steels is the Linear Polarization Resistance (LPR) method. The results of this test method provide measurements of corrosion current (Icorr,) or corrosion penetration rates. From this measurement, metal loss predictions can be made. Based on scientific formulations (Faraday’s Law), the amount of iron or steel reverting to rust at the time of testing is projected. Based on known amounts of rust gain in the deterioration process, section loss can be converted to rust accumulation at the steel. As the accumulation of rust will eventually lead to distress of the concrete, this data is vital to predicting performance for reinforced concrete structures and for use in durability modelling.
Introduction First introduced in 1957 by Milton Stern and A. L. Geary (Stern-Geary Equation), Linear Polarization Resistance (LPR) is an electrochemical technique used to measure the corrosion rate of a metal in an electrolyte. The object of a corrosion rate measurement in reinforced concrete is to determine the rate of embedded steel turning into rust. The corrosion rate measured in the field is a snapshot of the steel’s behavior at the time of measurement. The objective of the measurement is to be able to calculate, through Faraday’s Law of Metal Loss, what the average corrosion rate would be over a one year period based on the time the measurements were taken. These rates can then be utilized to determine long term performance of metals. Prior to taking the corrosion rate measurement, the Corrosion Potential, or Ecorr, is measured and forms the baseline of the corrosion rate test. The potential is then changed anodically (more positive) or cathodically (more negative) by 20 milliVolt (mV), where the resulting corrosion current, or Icorr, is measured maintaining the 20mV potential change. This polarizes the steel to determine the resulting current. Essentially, a controlled voltage change determines the amount of current needed to balance the corrosion reactions. The accumulation of scale is determined from measuring the amount of steel dissolving and forming oxide (rust). This is carried out by determining the electric current generated at the anodic reaction. The anodic reaction is where iron looses two electrons: Fe → Fe2+ +2e-
Equation 1
Structural teSting issues and advances related to structural testing
Figure 1. Michael Faraday, English Chemist and Physicist, c 1850s. Courtesy of Pictorial Collection, Science & Society Picture Library.
These electrons are then consumed at the cathodic reaction: The cathodic reaction is where hydrogen combines with oxygen and the two electrons lost at the anode are gained at the cathode site, to form hydroxyl ions: H2O + O2 + 2e- → 2OH-
Understanding the Rates of Corrosion in Concrete Structures Equation 2
The resulting data from the field testing provides the investigator with the steel potential (mV) and consequently the corrosion rate in microns per year (μm/yr). This has a direct relationship to metal loss which is calculated by Faraday’s Law.
Faraday’s Law The amount of material lost at the anode or deposited at the cathode is a function of the atomic weight of the metal or substance, the number of charges transferred, and the corrosion current (Icorr). This relationship was developed by Michael Faraday while working at the Royal Institute in London, England in 1833 (Figure 1).
Corrosion Rate Thresholds for Steel Reinforced Concrete Structures The deterioration rate is very important, as it enables the engineer to decide when minor or more major work needs to be made to the structure. In the use of coatings, the corrosion rate readings can identify when or how effective they are and can provide the engineer the option of applying further coatings when the corrosion rates start to increase to unacceptable levels. continued on next page
STRUCTURE magazine
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The Use of Corrosion Rates to Predict Material Performance By Gina Crevello, Irene Matteini and Paul Noyce Gina Crevello is material conservator and Principal of Echem Consultants. Gina is on the board of the Association for Preservation Technology International. She may be reached at gcrevello@e2chem.com. Irene Matteini is an architect and material conservator with Echem Consultants. She can be reached at imatteini@e2chem.com. Paul Noyce is a concrete and material durability expert. Paul is the Chairman of National Association of Corrosion Engineer’s (NACE) Standard Technical Group 01 for Reinforced Concrete. He may be reached at pnoyce@e2chem.com.
