What Does Monolithic Isolating Joint (MIJ) Mean?
A monolithic isolation joint (MIJ) is designed to act as a single piece (monolithic) electrical isolation (blocks cathodic protection system voltages) component that is welded or joined (joint) to the pipeline.
When designed and built properly, MIJs act as an integral part of the pipeline itself and provide long term integrity where cathodic protection (CP) Isolation is needed. While they perform the same duty within the CP system, flanged connections are isolated by gasket, sleeves and washers and all parts are visible to the naked eye - MIJ isolating components are completely encased in the steel forging bodies at the center of the design.
This creates a very robust method of isolating as it is not vulnerable to external attack from the environment or tampering, for example. For this reason, MIJs are most commonly used in buried applications and/or extreme or vulnerable environments above ground.
Corrosionpedia Explains Monolithic Isolating Joint (MIJ)
MIJs offer additional value beyond the fully encased monolithic design. One of the major benefits stems from the length of the MIJ as compared to the thickness of an isolating gasket between two flanges.
While a gasket would never be thicker than a fraction of an inch, an MIJ can be 4 feet or longer. By coating the ID of an MIJ, ‘long path isolation’ can be achieved which provides a defense against debris in the media building up on the ID surface of the isolating component. This is especially helpful in applications with conductive media or heavy build up of iron sulfides etc. in the line.
MIJs are designed to match the metallurgy and application demands of the pipeline in such a way that once they are installed, they will last the life of the line pipe they are joined to. Each MIJ should be designed and built to the specifications of the project whenever possible. This will ensure a long service life. If this is done properly, the MIJ will operate maintenance free for many years.