Hi, WoW
My first thought was this is probably not the only place the issue exists at your site.
Odds are there are other similar issue locations, unless you know exactly what caused the corrosion and therefore why the problem might be just in this one spot.
To your question: (In my Root Cause Analysis education/training it is heavily geared to Defining the Problem, specifically)
With much respect I would suggest taking a Big Picture look at this situation in a regulated environment.
First, is the issue Acute or Chronic. In other words is this an immediate Safety Issue requiring An immediate Shutdown or a Problem that needs resolution.
This is the path I would suggest
Contact Management, describe the finding, indicate you will be speaking w/ your Safety Department
Discuss the Hazard and possible injuries to individuals and damage to assets
Quickly determine what needs to be done immediately, cordon off area, signage, determine a safe way to cover the exposure (a hut?)
Inspect the rest of the facility, similar areas (underground), Is this systemic across the site or local to just here
Depending on findings
Systemic: You need help, it would be a huge undertaking to fix, electrical contractors, Digging Excavation contractors
Local: I believe at some point you will have to shut down. So do everything possible to make the time period as short as possible. Possibly create a parallel path(s) for the circuit(s). Installing junction boxes either side of the effected area. Wire box to box in prep. Prepare landing points for Old Wires to connect into New Junction Box Circuit, Once everything new/parallel is prepped safely, and a proper, safe, orderly shut down is performed, the switch over would hopefully go quickly.
Just my 2Cents
