Corrosion Basics – NACE International https://nace.org/resources/general-resources/corrosion-basics
This is a great resource for expanding on basic corrosion knowledge and access additional resources on corrosion prevention and mitigation methods. It contains general information on what corrosion is, factors to consider, common forms of corrosion and the importance of corrosion prevention.
Corrosion – A natural but controllable process
By Gretchen A. Jacobson – Materials Performance Managing Editor
Basic Corrosion CD-ROM Study Manual © NACE International, 2000
Methods of Corrosion Control 6:28
Cathodic and Anodic Protection
Cathodic and anodic protection are electrochemical techniques used for corrosion control. Cathodic protection has wide use in a variety of environments, including liquid immersion and soil. Anodic protection has a more limited, but important, application in chemical environments.
Principles
Cathodic protection reduces or eliminates corrosion by making the metal a cathode by means of an impressed current or attachment to a galvanic anode (usually magnesium, aluminum, or zinc).
The cathode in an electrochemical cell is the electrode where reduction (and no corrosion) occurs. Before cathodic protection is applied, corroding structures will have both cathodic areas and anodic areas (those areas where corrosion is occurring). If all anodic areas can be converted to cathodic areas, the entire structure will become a cathode and corrosion will be reduced.
How Cathodic Protection Works
Corrosion occurs where (positive) current discharges from metal to electrolyte at an anode. The objective of cathodic protection is to force the entire surface to act as a cathode, with current entering the environment and limited corrosion occurring,
The application of direct current to a corroding metal structure can cause it to become entirely cathodic.
To begin with, direct current is associated with the corrosion process on a buried or submerged metallic structure. This is illustrated in Figure 6.6.
As shown, direct current flows from the anodic areas into the soil, through the soil, onto the cathodic areas, and back through the pipe itself to complete the circuit. For a given driving voltage (the galvanic potential between anode and cathode), the amount of current flowing is limited by such factors as the resistivity of the environment (expressed normally in ohm-centimeters) and the degree of polarization at the anodic and cathodic areas.
Corrosion occurs where the current discharges from metal into the electrolyte (soil) at anodic areas. Where current flows from the environment onto the pipe (cathodic areas), there is no corrosion. In applying cathodic protection to a structure, the objective is to force the entire surface exposed to the environment to be cathodic to the environment. When this condition has been attained, corrosion is mitigated.
Galvanic Anode Cathodic Protection Systems
In the galvanic anode system, pieces of an active metal, such as zinc or magnesium, are placed in contact with the corrosive environment and are electrically connected to the structure to be protected. The galvanic anodes corrode preferentially, providing protection to the structure.
A galvanic (sacrificial) anode can be described as a metal that will have a voltage difference with respect to the corroding structure and will discharge (positive) current that will flow through the environment to the structure.
The basic configuration of a galvanic cathodic protection system is shown in Figure 6.7.
Impressed-Current Cathodic Protection Systems
In an impressed-current cathodic protection system, an external source of direct-current power is connected (or impressed) between the structure to be protected and the ground bed.
With an impressed-current system, the ground bed anodes are not depended upon as a source of electrical energy. Instead, an external source of direct-current power is connected (or impressed) between the structure to be protected and the ground bed.
The basic configuration of an impressed-current cathodic protection system is shown in Figure 6.8.
The positive terminal of the power source must always be connected to the groundbed, which is then forced to discharge as much cathodic protection current as is desirable. If a mistake is made and the positive terminal is connected to the structure to be protected, the structure will become an anode instead of a cathode and will corrode actively.
SP0388-2014 (formerly RP0388) Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Storage Tanks
SP0169-2013 (formerly RP0169), “Control of External Corrosion on Underground or Submerged Metallic Piping Systems”
SP0196-2015 (formerly RP0196), “Galvanic Anode Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks”
ASTM G95 – 07(2013) Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings (Attached Cell Method)
ASTM G95-07(2013), Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings (Attached Cell Method), ASTM International, West Conshohocken, PA, 2013, www.astm.org
http://www.astm.org/cgi-bin/resolver.cgi?G95
NACE ASTM G193-12D Standard Terminology and Acronyms Relating to Corrosion, NACE International, Houston, TX, 2012, www.nace.org, https://www.nace.org/resources/industry-standards
NACE SP21412-2016/SSPC-CPC 1, Corrosion Prevention and Control Planning, NACE International, Houston, TX, 2016, www.nace.org, https://www.nace.org/resources/industry-standards