EXAMPLES OF CORROSION RESOLUTIONS CORROSION MONITORING METHODS

METHOD

TYPE OF DATA PRODUCED

BASIC LIMITATION OF THE METHOD

Visual/Mechanical Measurements Total penetration to date 1.  Can only be used during shutdown/partial outage or when, e.g. a spoolpiece can be provided
2.  Does not reflect the present corrosion rate and may not reflect the present corrosion mechanism
Coupon Immersion and/or Spoolpiece Analysis Long term performance data on uniform corrosion and some forms of localized corrosion 1.  Provides time weighted average corrosion rate.
2.  Cannot be used as an early warning system.
3.  Substantial intermittent manpower required.
Destructive Analysis, e.g. Metallography, EDS, etc. * Corrosion mechanism
* Corrosion rate
Intermittently labor intensive
Ultrasonic Testing Total uniform and localized corrosive penetration. 1.  Provides time weighted average corrosion rate.
2.  Surface preparation may be required.
3.  Substantial intermittent manpower required.
Eddy Current Testing - Lift Off Technique Total pitting penetration and location of corrosion Substantial intermittent manpower required.
Electrical Resistance Technique Time weighted average corrosion rate with an indication of trends in corrosion rate. 1.  Does not necessarily reflect the present-day corrosion rate.
2.  Useful data produced only when uniform corrosion or erosion-corrosion predominates.
3.  Only fair performance as an early warning device
Hydrogen Penetration Hydrogen penetration through metal lattice, indication of embrittlement failure susceptibility and uniform corrosion rate. Limited usefulness, except where potential hydrogen embrittlement is a concern.
AC Impedence * Corrosion mechanism
* Corrosion rate
Intermittently labor intensive.
Bioassay & Spectroscopic or Other Analyses * Identification of microorganisms involved in microbiologically influenced corrosion & confirmation of activity by spectroscopic & other analytical methods which identify the necessary presence of by-products, metabolites, etc.

* Chemical analysis of coolant and/or corrosion product for various analytical reasons.
Intermittently labor intensive.
Linear Polarization Resistance or Tafel Slope Extrapolation Operational uniform corrosion rates Not applicable to demineralized water systems -   electrolyte required.
Open Circuit Corrosion Potential Monitoring Indication of susceptibility to localized corrosion Not quantitative
Full Polarization & Cyclic Potentiodynamic Hysteresis Analysis * Corrosion Mechanism
* Corrosion Rate
Must be used infrequently; therefore, cannot be used as an early warning device.