Corrosion on the outer surface of insulated pipes, vessels and tanks, caused by water that gets through the insulation and is held against the metal. Zoom from the pipe down to the wet steel, where it happens.
At the anode, iron leaves the metal as positively charged ions and gives up electrons. The electrons travel through the pipe wall to the cathode, where they are consumed, typically by oxygen and water.
PHMSA 2008Steel corrodes when it meets water with a supply of oxygen. HSE 2011
The water is the electrolyte: it carries ions and closes the circuit. Take it away and the cell stops. PHMSA 2008
Fibrous mineral wool, fibreglass and calcium silicate soak up water easily; closed-cell foam glass and perlite take up less. HSE 2011
Held against the metal, the film persists. Let it dry and, once it has gone, so has the electrolyte.
Frequent swings between running and shutdown rewet the insulation. HSE 2011Cao et al. 2022
Off: the line holds one condition.
* citing NACE SP0198 and API RP 583. Bands as the two sources give them; the page reads peak risk where they overlap, about 50 to 120 °C.
Operating temperature has a strong effect on risk. HSE 2011Cao et al. 2022
In the view, heat changes only the film. Above 120 °C HSE expects moisture to dry out at start-up; lines above about 150 °C once stayed dry because moisture boiled off. HSE 2011Cao et al. 2022 No corrosion rate is modelled. The page reports about 0.5 mm a year at 80 °C under insulation (HSE, citing US data for carbon-manganese steel) and one case of about 1 mm a year at 60 to 120 °C (Cao et al.).
A metal can only oxidize if something else is reduced at the same time. In the view, iron leaves only while the cathode takes electrons. NASA KSC
The liquid over an anodic spot turns more acidic, so once a pit starts, the attack keeps concentrating there. PHMSA 2008
The anode and cathode may be right next to each other or several feet apart. PHMSA 2008