The failure that only exists behind the cladding

Corrosion under insulation is the least visible failure mode in a plant because the evidence is covered by the very material installed to manage heat. Water gets into the insulation, sits against the metal wall, evaporates and returns with every weather cycle, and the chloride and dissolved solids it carries concentrate rather than wash away. General practice puts the worst damage between roughly minus 4 and 175 degrees Celsius on the metal surface, which is a wide band that covers most process piping, most storage vessels and a lot of equipment that nobody considers "cold service". The steel plate surface treatment page lists the protective layers that decide how long a carbon wall lasts in exactly this condition, and the rest of the specification is about what happens when those layers are breached.

Why the water never leaves

Insulation is not a waterproof jacket. Cladding laps breathe, banding punctures, and foot traffic breaks seals, so the inside of the system sees rain, washdown, steam leaks and condensation. When the wall is warm the water dries; when it is cool or the line cycles, the residue stays. What matters is that the liquid trapped at the interface is far more aggressive than the ambient rain outside, because dissolved solids accumulate at the point of evaporation. That is why under-insulation attack is local rather than general, and why a wall that looked sound at the last shutdown can be thinned through at the support rings by the next one.

The carbon side: paint, zinc and honest replacement stock

On carbon equipment the coating is the corrosion barrier, and the catalogue treats it that way. The surface treatment page is explicit about the sequence: rust removal by sandblasting, shot blasting or pickling first, then a coating layer to isolate air and moisture, with galvanizing offered as the common anti-corrosion treatment that forms a protective zinc layer on the surface. For structural steel that never gets stripped and inspected, hot dip galvanized H-beam carries a zinc-iron alloy layer bonded to the steel rather than a paint film that ages, and the Z275 galvanized coil page shows the spec range behind that choice: 0.12 to 3 mm substrate with zinc coating from Z60 through Z80 to 275 g/m2. Fabricated supports, ladders and cladding frames go through the galvanized processing route of CNC laser cutting, metal bending, welding and polishing, zinc coating, then assembly and packing.

When the lagging does come off and the wall has to come out, the replacement stock is ordinary plate. Q235 plate, A36 and Q345 are listed from 0.3 to 200 mm in widths of 1000, 1250 and 1500 mm, which covers patch plates and shell courses without exotic procurement. Cutting, bending and welding are standard services on those pages, and a repair that is properly coated before it is re-insulated lasts the life of the surrounding system.

Where the hot lines live

Steam tracing is a special case because the traced line is warm enough to stay dry most of the year and wet enough to corrode whenever the tracing fails. The pressure side of that world is A106 seamless pipe, which is hydrostatically tested on every length with a nondestructive electric test as the alternative, and the structural side is A53. Both tolerate a coating defect far longer than they tolerate a wet one, which is the whole argument for inspecting tracing before winter rather than after.

The stainless trap: the outside of the wall is not the inside

Austenitic stainless buys no automatic immunity here, and this is where specification mistakes are expensive. External chloride stress corrosion cracking needs three things the insulation provides: chlorides leached from the insulation or the atmosphere, moisture, and tensile stress left in the pipe by welding and bending. The 304/304L coil page shows a nickel range of 8.0 to 10.5 percent, and the 316/316L coil adds 2 to 3 percent molybdenum for pitting, but neither changes the chloride cracking arithmetic much at the wet-dry line. The stainless pipe and 304/304L pipe pages list the product forms that spend their lives under lagging, and the practical answer is stress control and inspection access, not a thicker wall.

Where chlorides are high and the temperature band overlaps, the nitrogen-strengthened and duplex grades earn their premium. 317LMN is the inexpensive step up, stocked in sheet and plate as the 317LMN plate and coil listing shows. 254 SMO is the super austenitic answer to high chloride process streams, and 904L carries sulphuric and phosphoric media as well as seawater. On the duplex side the 2205 page is candid about its window: particularly suitable from minus 50 to plus 600 degrees Fahrenheit, with yield strength about twice that of austenitic grades, available as S31803 seamless pipe and S32205 pipe. 2507 is limited below 600 degrees Fahrenheit or 316 Celsius, the S32760 page publishes PREN 40 with NACE MR 01-75 listing, and LDX 2101 is the economy grade with 65 ksi minimum yield. The duplex coil listing carries 0.2 to 60 mm by up to 2000 mm wide in finishes from No.1 to 2B and No.8, with a one tonne minimum order and seven to fifteen working days after deposit, so a selective upgrade is a procurement decision, not a project.

Nickel and titanium where the pockets concentrate

Certain locations deserve the top of the ladder: dead legs under insulation, the wet-dry line on vents and drains, and anything that also sees acid. The 825 page stocks plate from 4.8 to 50.8 mm and describes resistance across oxidising and reducing acids including alkalis; Alloy 600 is approved under ASME Code Sections I, III and VIII; Monel 400 resists neutral and alkaline salts; C-276 withstands wet chlorine gas and hypochlorite; and the 625 page claims strength from cryogenic to 2000 degrees Fahrenheit. The titanium rod page documents seawater corrosion below 0.005 mm per year, a twentieth of 316L.

The hot end of the same problem

Above the wet band the chemistry changes and the standard grades change with it. 310S is published for continuous service to 2000 degrees Fahrenheit, the 310S pipe listing carries the product form, and the stabilised grades take the intermediate band: 321 is titanium stabilised for 1000 to 1600 degrees Fahrenheit with an optional stabilising treatment at 1550 to 1650, while 347H pipe carries 515 MPa tensile and 205 MPa yield. Large bore replacement sections come from the large diameter thick wall listing, frames and platforms from square stainless tube and the steel structure page, and shop work from the cutting and bending services.

What actually goes on the enquiry

Four lines decide most of this. The operating and design metal temperatures, because the corrosion band and the alloy window both live on temperature. The insulation type and its chloride specification, because the leachate is the corrosive agent. The coating system for carbon surfaces and the stress state for stainless weldments. And the inspection plan, because the only way to manage a failure mode that hides behind cladding is to decide in advance which sections get opened. Specify those four and the material choice stops being a debate and becomes arithmetic.