Corrosion Science & Industry Terminology Glossary

Corrosion Science & Industry Terminology Glossary

The Importance of Terminology

To succeed on the AMPP CP1 exam and in your career as a cathodic protection tester, you must become fluent in the language of corrosion science and electricity. Misunderstanding a single term in a test question can lead you to the wrong answer. This section provides a detailed glossary of the most critical terms you must know. We will break these down into two main categories: Corrosion Terminology and Electrical Terminology.

Core Corrosion Terminology

The Corrosion Cell

Corrosion of metals in aqueous environments is an electrochemical process. For this process to occur, a corrosion cell must exist. A corrosion cell consists of four mandatory components. If any one of these is removed, corrosion stops.

1. Anode: The electrode of a corrosion cell where oxidation occurs. This is the area where metal is lost (corrodes). At the anode, metal atoms lose electrons and become positive ions that dissolve into the electrolyte.
2. Cathode: The electrode of a corrosion cell where reduction occurs. This is the area that is protected from corrosion. At the cathode, electrons arrive through the metallic path and are consumed in chemical reactions at the surface.
3. Electrolyte: A chemical substance containing ions that migrate in an electric field. For CP purposes, the electrolyte is usually soil or water. The electrolyte conducts ionic current (the movement of ions) between the anode and cathode.
4. Metallic Path (Return Circuit): The physical connection between the anode and cathode that allows electrons to flow from the anode to the cathode. This is usually the pipeline or structure itself, or a wire connecting different parts of the structure.

Electrochemical Reactions

• Oxidation: A chemical reaction in which a substance loses electrons. In corrosion, oxidation always occurs at the anode. (Remember the mnemonic: AN OX - Anode Oxidation).
• Reduction: A chemical reaction in which a substance gains electrons. In corrosion, reduction always occurs at the cathode. (Remember the mnemonic: RED CAT - Reduction Cathode).
• Ion: An electrically charged atom or group of atoms. Cations are positively charged (e.g., $Fe^{2+}$), and anions are negatively charged (e.g., $Cl^-$).

Cathodic Protection Terms

• Cathodic Protection (CP): A technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell.
• Galvanic (Sacrificial) Anode: A metal that provides sacrificial protection to another metal that is more noble when electrically coupled in an electrolyte. For example, zinc or magnesium attached to a steel pipe.
• Impressed Current Cathodic Protection (ICCP): A CP system that utilizes an external power source (usually a rectifier) to drive direct current from an anode (e.g., mixed metal oxide) to the structure to be protected.
• Reference Electrode (Half-Cell): A device whose open-circuit potential is constant under similar conditions of measurement, used for measuring the relative potential of other electrodes. The Copper/Copper Sulfate Electrode (CSE) is the most common in soil.
• Structure-to-Soil Potential: The voltage difference between a buried metallic structure and the electrolyte, measured using a reference electrode placed in the soil.

Core Electrical Terminology

Understanding electrical terms is vital because CP involves manipulating electrical currents to stop corrosion.

• Voltage (Electromotive Force, E or V): The electrical pressure or driving force that pushes current through a circuit. It is measured in Volts (V) or millivolts (mV). Voltage is the difference in electrical potential between two points.
• Current (I): The rate of flow of electrical charge. It is measured in Amperes (A) or milliamperes (mA).
• Resistance (R): The opposition to the flow of electrical current. It is measured in Ohms ($\Omega$). All materials have some resistance, though it varies widely.
• Direct Current (DC): Electrical current that flows continuously in only one direction. Cathodic protection utilizes DC.
• Alternating Current (AC): Electrical current that reverses its direction of flow at regular intervals (e.g., 60 times a second for standard US power). Rectifiers convert AC power to DC power for ICCP systems.

Current Flow Direction

This is a concept that frequently trips up candidates. There are two ways to describe the flow of electricity:

• Electron Flow: The actual physical movement of electrons. Electrons are negatively charged, so they flow from the negative pole to the positive pole. In a corrosion cell, electrons flow through the metallic path from the Anode to the Cathode.
• Conventional Current: A historical convention established before the discovery of the electron. It assumes current flows from the positive pole to the negative pole. In the CP industry, unless specifically stated otherwise, we usually speak in terms of conventional current. Therefore, conventional current leaves the Anode, travels through the electrolyte, and enters the Cathode.

Key Exam Tip: Always read questions carefully to see if they are asking for the direction of electron flow or conventional current flow.

Measurement and Circuit Terms

• Series Circuit: A circuit where components are connected end-to-end, providing only a single path for current flow. The current is the same everywhere in a series circuit.
• Parallel Circuit: A circuit where components are connected across each other, providing multiple paths for current flow. The voltage is the same across all branches of a parallel circuit.
• Short Circuit: An abnormal connection of relatively low resistance between two points of different potential in a circuit, often causing a massive increase in current flow.
• Isolation (Dielectric) Joint: A fitting used to electrically separate two sections of a metallic structure (e.g., an insulating flange on a pipeline). This is essential for controlling CP current and preventing it from flowing to unintended structures.

By mastering these terms, you will be much better equipped to understand the complex scenarios presented on the CP1 exam. Refer back to this glossary whenever you encounter a term you are unsure about during your studies.