suspendedn ear the wall of the vessel or pipe, for example. Since corrosive
conditions can significantly changefrom one location to another,coupon data
is best used for relative comparisons (changesinthe rate or characteristics of
corrosion over time at asingle point) and to obtain an approximate corrosion
rate at aparticular point in the system ratherthan to precisely calculate the
corrosion rate.
There are several othertypes of couponsavailable for specialized analysis.
These include disk couponst hat can be mounted flush to the pipe wall;
ladder coupon holders for mounting couponsatseveral depths at the same
point in the piping; prestressed couponst hat are for the investigation of
possible stress cracking mechanisms; and welded coupons that are used to
detectpreferential corrosive action on weldments.
11.1.7H ydrogen Probes
Hydrogen probes are used to detectt he penetrationofe lemental hydrogen
into metal, pipe, andv essel. This can occur in cathodic reactions in acid
solutions, particularly in the presence of hydrogen sulfide. There are three
basic types of hydrogen probes. The simplest and most common consists of
at hin-walled carbons teel tube inserted into the flow stream with as olid
rod inside the tubef orming as mall annular space. Hydrogen atoms small
enough to permeate the carbon steel collect in the annular space and combine
to form molecular hydrogen gasthat is toolarge to pass back into the process.
As hydrogen gascollects, pressure builds in the annular spaceand registers
on ap ressureg auge located outside the piping. “Patch probes” operate
identically except that the “patch” is sealed to the outside of the pipe or vessel
and collects hydrogen atoms that penetrate the pipe wall. The third type of
hydrogen probe is the palladium foil type that produces an electrical output
proportional to hydrogen evolution rate.
All three types of hydrogen probes provide useful information about
changesint he corrosive environment in processes where HIC is present or
could occur.I fp ressure is accumulating at ac ertain rate per day and then
increases by af actor of 10, this indicates as ignificant changet hat should
be addressed. Hydrogen probe readings, however,h ave not been found
to be suitablef or direct determination of corrosionr ate, only of changes in
the process, and it is recommend that they be applied carefully as part of
acomplete corrosion controlprogram in applications where HIC is aconcern.
11.1.8P olarization Studies
Polarization studies have been primarilyl aboratory electrochemical techniquest os tudy corrosion phenomena, especially pitting, by disturbing the
natural corrosion potential of as ystem, frequently by substantial amounts
(a fewv olts compared with af ew millivolts in linear polarization
measurements) and measuring the external current flowing. With advances
in computert echnology,s omeo ft hese systems are now beingu sed in
the field. In the laboratory,t he metal/fluid environment can be carefully
Corrosion Monitoring
537
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
conditions can significantly changefrom one location to another,coupon data
is best used for relative comparisons (changesinthe rate or characteristics of
corrosion over time at asingle point) and to obtain an approximate corrosion
rate at aparticular point in the system ratherthan to precisely calculate the
corrosion rate.
There are several othertypes of couponsavailable for specialized analysis.
These include disk couponst hat can be mounted flush to the pipe wall;
ladder coupon holders for mounting couponsatseveral depths at the same
point in the piping; prestressed couponst hat are for the investigation of
possible stress cracking mechanisms; and welded coupons that are used to
detectpreferential corrosive action on weldments.
11.1.7H ydrogen Probes
Hydrogen probes are used to detectt he penetrationofe lemental hydrogen
into metal, pipe, andv essel. This can occur in cathodic reactions in acid
solutions, particularly in the presence of hydrogen sulfide. There are three
basic types of hydrogen probes. The simplest and most common consists of
at hin-walled carbons teel tube inserted into the flow stream with as olid
rod inside the tubef orming as mall annular space. Hydrogen atoms small
enough to permeate the carbon steel collect in the annular space and combine
to form molecular hydrogen gasthat is toolarge to pass back into the process.
As hydrogen gascollects, pressure builds in the annular spaceand registers
on ap ressureg auge located outside the piping. “Patch probes” operate
identically except that the “patch” is sealed to the outside of the pipe or vessel
and collects hydrogen atoms that penetrate the pipe wall. The third type of
hydrogen probe is the palladium foil type that produces an electrical output
proportional to hydrogen evolution rate.
All three types of hydrogen probes provide useful information about
changesint he corrosive environment in processes where HIC is present or
could occur.I fp ressure is accumulating at ac ertain rate per day and then
increases by af actor of 10, this indicates as ignificant changet hat should
be addressed. Hydrogen probe readings, however,h ave not been found
to be suitablef or direct determination of corrosionr ate, only of changes in
the process, and it is recommend that they be applied carefully as part of
acomplete corrosion controlprogram in applications where HIC is aconcern.
11.1.8P olarization Studies
Polarization studies have been primarilyl aboratory electrochemical techniquest os tudy corrosion phenomena, especially pitting, by disturbing the
natural corrosion potential of as ystem, frequently by substantial amounts
(a fewv olts compared with af ew millivolts in linear polarization
measurements) and measuring the external current flowing. With advances
in computert echnology,s omeo ft hese systems are now beingu sed in
the field. In the laboratory,t he metal/fluid environment can be carefully
Corrosion Monitoring
537
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547