Corrosion Monitoring

EDS 2004/Metallurgy 9-1

Corrosion Monitoring


Outcomes

Recall classes of corrosion monitorsIdentify operating principlesIdentify significant limitations


Types of Corrosion Monitors

NDTCorrosion couponsElectrical resistance probesLinear polarization resistance probesHydrogen probes


Non Destructive Testing


Nondestructive Testing

Advantages and disadvantages vary somewhat with methodAdvantages– Direct measurement of metal loss– Equipment is portable so measurements can be

made at any desired location– May be usable on stream– Portable – move until corrosion is found


Nondestructive Testing

Disadvantages– May or may not show pitting or cracking– May or may not show corrosion around

complex geometries– Gives only an average corrosion rate


Corrosion Coupons


Corrosion Coupons

Advantages– Direct evidence of corrosion– Shows pitting– Can show cracking, crevice corrosion, and

galvanic corrosion– Can be conducted on line or in lab– Can be used in any corrosive environment– Available in essentially all commercial alloys


Corrosion Coupons

Disadvantages– Gives only an average corrosion rate– Must select proper location in process– Intrudes into process– Improper cleaning/handling will give

erroneous results– SLOW


Corrosion Coupons


Corrosion Coupons


Corrosion Coupons

Greatest benefit of using corrosion coupons is for the selection of a new alloy in a process known to be corroding.


Electrical Resistance Probe



Advantages– Fast enough to detect the effects of process

changes on corrosion rate (sensitivity on the order of 1 - 2 days)

– High sensitivity probes reported to respond more rapidly

– Work in most corrosive environments— some limits on corrodants and temperature



Advantages– Available in most commercial alloys– May detect pitting



Disadvantages– Must select proper location in process– Time sensitivity insufficient to measure the

effect of upsets on corrosion– May protrude into process– Must be changed periodically


Electrical Resistance Probes



New

Not so New

Electrical resistance of probe varies with cross section of wire





Typical electrical resistance probe sensitivity


Linear Polarization Probe



Probe is electrochemical in nature– Applies a small 10 - 20 mV sinusoidal potential– Measures current response– Calculates resistance of system– Calculates corrosion rate based on resistance



Current i(measured)

Potential εε = Eapp - Ecorr

Slope = = Rp∆ε∆i

∆ε∆iRp = = B

icorr

B is a constant for a given system and relativelyconstant between systems



Advantages– Speed

— Test are short— Responds quickly enough to detect corrosion

caused by upsets– Electrodes available for most commercial alloys– Will normally detect pitting



Disadvantages– Requires at least some electrolyte– Beneficial to determine B independently or to

calibrate with coupon tests– Must select proper location in process



Probes can be 2 or 3 electrode




Hydrogen Permeation Sensor


Hydrogen Permeation

Indicator for– Aqueous corrosion when certain species are

present e.g. HF, H2S, CN-, As– Hydrogen embrittlement– Hydrogen blistering– SOHIC– Sulfide stress cracking


Pressure Gauge

Installed at a fixed locationHydrogen penetrates the tip of the gauge and is trapped. Pressure increases within the system and is recorded on the gaugeThe gauge is periodically reset


Pressure Gauge


Hydrogen Sensor

“Patch” attaches to pipe or vessel– Magnetic or Strap– Low or high temperature

Air pulled through channels in patch– Sweeps hydrogen to remote sensor

Sensor measures hydrogen concentration– Hydrogen flux calculated

Portable – can move to find corrosion


Hydrogen Sensor

Useful for– High temp corrosion e.g. naphthenic acid– Predicting/measuring hydrogen

blistering/sulfide stress cracking– May or may not predict general corrosion rates

— Effect of recombination poisons on hydrogen penetration must be calibrated


Hydrogen Sensor

Corrosion Monitoring

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