API 653 Cathodic Protection Systems 2

Question 1

What is stray current corrosion in the context of cathodic protection systems for storage tanks?

Accepted Answer

Corrosion caused by electrical currents from external sources flowing through the tank structure

Answer

Stray current corrosion occurs when electrical currents originating from external sources (such as DC transit systems or neighboring CP systems) flow through the tank structure and discharge at unintended locations, causing accelerated corrosion.

Question 2

What test is performed to evaluate whether a cathodic protection system is providing adequate protection at a tank bottom?

Accepted Answer

Pipe-to-soil potential survey

Answer

A pipe-to-soil (or structure-to-soil) potential survey measures the electrochemical potential of the tank bottom relative to a reference electrode in the soil to determine whether the -850 mV (CSE) protection criterion is met.

Question 3

What is the purpose of a 'close-interval potential survey' (CIPS) for an aboveground storage tank bottom?

Accepted Answer

To obtain detailed potential readings at small spacing intervals around the tank perimeter to identify CP shielding or deficiencies

Answer

A close-interval potential survey provides detailed potential measurements at small spacing intervals, allowing identification of areas where CP protection may be deficient or where CP current is being shielded.

Question 4

Which of the following would indicate OVERPROTECTION by a cathodic protection system on a coated steel tank bottom?

Accepted Answer

Potentials more negative than -1200 mV vs. CSE

Answer

Potentials more negative than approximately -1200 mV vs. CSE indicate overprotection, which can cause hydrogen embrittlement of high-strength steel or disbondment of coatings due to excessive hydrogen generation.

Question 5

Why is electrical isolation (isolation joints/flanges) important in a cathodic protection system for a storage tank?

Accepted Answer

To prevent CP current from being drained by connected pipelines or structures not intended to be protected

Answer

Electrical isolation prevents CP current from being diverted to connected metallic structures (such as piping systems) that are not part of the intended protection zone, ensuring efficient use of CP current on the tank bottom.

(API 653) API 653 Aboveground Storage Tank Inspector Certification Practice Test

(API 653) API 653 Aboveground Storage Tank Inspector Certification Practice Test

API 653 Cathodic Protection Systems 2