boothuk
Petroleum
- Jul 14, 2010
- 12
Hello,
I am a coating's technologist currently handling a huge throughput of cathodic disbondment testing (CDT).
We are a UKAS approved laboratory for this test, i.e. we must maintain everything according to specification for the duration of the test.
I am seeking suggestions for the most apt chemical to neutralise the solution in the CDT cell as the pH rises (please see below).
(1) 2H+ + 2e- ?? H2
(2) H2O + ½ O2 + 2e- ?? 2OH-
(3) Fe2O3 + 3 H2O + 2 e- ?? 2Fe(OH)2 + 2 OH-
(4) H2O + e- ?? ½ H2 + OH
Reduction
of hydrogen ions (equation 1) from the aqueous electrolyte under the influence of an
electrical current quickly increases pH (making the solution at the cathode more alkaline). As the pH rises, the
reaction becomes unfavorable. pH increases through the reduction of oxygen from solution, forming more
hydroxyl ions (equation 2). Ferric oxide reduction (equation 3) on the steel surface to form ferrous hydroxide
results in a high pH environment in the crevice area (between the coating and the steel substrate) around the
periphery of the holiday. The formation of a silver halo on the steel surface around the holiday has been
attributed to a loss of cathodic potential beneath the disbonded coating, where oxide growth occurs as a result of
insufficient cathodic protection (cathodic shielding). Finally, water itself may be reduced and raise the local
pH, as illustrated in equation 4.
Some specifications, in particular the NFA, require that the 3%SS should be maintained between a pH of between 6.0 & 8.5.
Replacement of the electrolyte is not feasible due the the samples being held in a sandbed with cooling coils inserted.
I am a coating's technologist currently handling a huge throughput of cathodic disbondment testing (CDT).
We are a UKAS approved laboratory for this test, i.e. we must maintain everything according to specification for the duration of the test.
I am seeking suggestions for the most apt chemical to neutralise the solution in the CDT cell as the pH rises (please see below).
(1) 2H+ + 2e- ?? H2
(2) H2O + ½ O2 + 2e- ?? 2OH-
(3) Fe2O3 + 3 H2O + 2 e- ?? 2Fe(OH)2 + 2 OH-
(4) H2O + e- ?? ½ H2 + OH
Reduction
of hydrogen ions (equation 1) from the aqueous electrolyte under the influence of an
electrical current quickly increases pH (making the solution at the cathode more alkaline). As the pH rises, the
reaction becomes unfavorable. pH increases through the reduction of oxygen from solution, forming more
hydroxyl ions (equation 2). Ferric oxide reduction (equation 3) on the steel surface to form ferrous hydroxide
results in a high pH environment in the crevice area (between the coating and the steel substrate) around the
periphery of the holiday. The formation of a silver halo on the steel surface around the holiday has been
attributed to a loss of cathodic potential beneath the disbonded coating, where oxide growth occurs as a result of
insufficient cathodic protection (cathodic shielding). Finally, water itself may be reduced and raise the local
pH, as illustrated in equation 4.
Some specifications, in particular the NFA, require that the 3%SS should be maintained between a pH of between 6.0 & 8.5.
Replacement of the electrolyte is not feasible due the the samples being held in a sandbed with cooling coils inserted.
