The rear box is a wet vessel that happens to be hot

An exhaust system is bought as a hot component and spends most of its life cold. From manifold to tailpipe it carries hot gas for twenty minutes and holds weak acid for the other twenty-three hours, and the part that fails first is the rear muffler, where the gas has cooled enough to let its water out.

That decides the grade. A 409 stainless sheet is not chosen because it is cheap stainless; it is the least expensive metal that survives condensate.

One kilogram of petrol makes 1.4 kilograms of water

Octane burns to carbon dioxide and water: C8H18 + 12.5 O2 gives 8 CO2 + 9 H2O. By mass that is 114 g of fuel to 162 g of water: every kilogram of petrol puts 1.42 kg out of the tailpipe, and a car at 8 litres per 100 km makes 8.5 kg.

At temperature all of it leaves as vapour. On a six-kilometre commute the rear box rarely reaches 100 °C, so the vapour condenses, runs to the lowest point and stays: one to two litres after a week of short winter trips, against a skin at 60 to 90 °C — warm enough to speed the reaction, not hot enough to boil the water off.

What that water is carrying

Condensate is not distilled water. Fuel sulphur at 10 ppm leaves about 120 mg of sulphur dioxide from the same 6 kg of fuel; in the two litres that stay in the box that is roughly a millimole per litre of sulphurous acid, low threes on the pH scale. Nitrogen oxides add nitrate, ethanol blends add acetate, and road salt adds chloride by the hundred milligrams per litre, which turns general thinning into pitting.

Geometry matters as much. Condensate collects at the seam and the baffle weld, in a crevice with no oxygen and no drainage. A drain hole at the low point is worth more than a grade step, and it costs nothing.

Eleven per cent chromium and a titanium addition

The 409 grade carries 10.5 to 11.75 per cent chromium, 0.030 per cent carbon maximum and half a per cent nickel, with 380 MPa minimum tensile, 170 MPa yield and 20 per cent elongation. Eleven per cent is not a comfortable number; it is the floor at which a passive film forms at all, which is why the same page calls it the minimum for stainless behaviour.

The titanium is the part buyers forget. At six times the carbon and no more than 0.75 per cent, it ties carbon up as carbide before the metal is welded, so chromium stays in solution and the heat-affected zone does not sensitise. The page also lists the fully stabilised S40910, S40920 and S40930, stabilised with niobium, titanium or both. On a welded muffler that suffix is the specification: plain S40900 welded into a cold-end box sensitises in the 400 to 800 °C band behind the weld and rusts along it first.

Its pitting resistance equivalent is about 11, with no molybdenum and no nitrogen. It is not a chloride alloy; it is three to five times the life of carbon steel in mild condensate, and should be sold as that.

The weld is where the grade is decided

Ferritic stainless has no austenite to transform, so the heat-affected zone does not harden; it grows. Grains coarsen and toughness falls, leaving the weakest part of the box. Fillers are matched low — 409 where it can be had, 308L where it cannot — at low heat input and with a purge inside the tube.

The coloured oxide then has to come off. A heat tint on a 409 seam is a chromium-depleted film sitting in condensate, and it fails before the parent metal does. Pickling and passivation of the seam is a line item deleted in cost reviews and reinstated after the first warranty claim.

Why 304 is the wrong answer at the rear

The reflex upgrade is 304, and it fails on a number that has nothing to do with corrosion. Its mean expansion is 17.2 × 10⁻⁶/K against 11.7 for 409 over the same 0 to 315 °C range. Take a 1.4 m system heated 600 K: the austenitic grows 14.4 mm, the ferritic 9.8 mm, and the 4.6 mm difference has to be absorbed by hangers, a flex joint or a slip seam that then leaks.

Conductivity runs the same way — 25.8 W/m·K at 100 °C for 409 against 16.2 for 304 — so the ferritic box picks up heat along the pipe and dries sooner. Where the rear box genuinely sees brine, the answer is a molybdenum grade at the back only: 316L coil or a 304 pipe for the tail section, with the expansion paid for by a slip joint rather than ignored.

Aluminised steel and where it stops

Below 409 in cost sits aluminised steel, and its page is honest about the limit: cheaper than grade 409, but less resistant to exhaust gases. The aluminium-silicon coating is a barrier, not a sacrificial one. It holds while continuous and while the part stays dry, which is why it works on heat shields and hot-end shells but fails on a cold-end muffler where condensate sits against a cut edge.

Zinc is out of the question above 230 °C, where the Galvalume data puts ordinary hot-dip galvanized steel, and even that coating stops at 315 °C. A galvanized coil belongs on the bracket, not the shell.

Gauge, allowance and the hanger load

A 1.2 mm skin weighs 9.36 kg per square metre at 7,800 kg/m³, so a rear box 1.4 m long and 400 mm around carries about 5.2 kg plus baffles. At 0.1 mm a year in a bad cold end, with 0.3 mm held back for forming and handling, the wall gives about nine years. Another 0.3 mm buys three more years for 25 per cent more weight; 1.0 mm is a five-year part.

Hangers are sized on the hot case: a 9 kg box on two rods sees the growth of the system plus engine roll, and the rubber that isolates vibration hides a stretched rod until the box cracks at the strap weld.

What goes on the purchase order

Grade with UNS number and stabilised suffix, thickness and tolerance, width, finish, edge condition, mill certificate. "409" on its own does not say stabilised, and S40900 against S40930 is the difference between a weld that survives ten winters and one that does not. On a drawn shell the 410 sheet and 410S coil pages are the wrong shelf: martensitic grades harden in the weld and belong on brackets and flanges.

Brackets and shield frames are a separate order. 430 at 16 to 18 per cent chromium, 483 MPa tensile and 10.4 × 10⁻⁶/K expansion holds a shield through the same cycles without the nickel bill, usually bought as 430 strip. 321 is for the manifold end, where stabilisation under repeated heat cycles is not optional; 310S belongs to the turbine side of an engine, not to a road car.