High-humidity storage: why parts rust and how to stop it

A batch that passed dimensional and coating inspection can be rusty at the assembly line three weeks later, and the drawing will not explain why. The parts did not change. The climate around them did. In a monsoon warehouse, the air holds enough water to condense on any surface that falls a few degrees below the dew point, and a steel part sitting under a metal roof overnight does exactly that.

High-humidity storage is a sequence problem. It starts at the pretreatment line, runs through the hours a part spends bare or phosphated between operations, and ends in the packing chosen for the crate. Get the sequence wrong and the coating is blamed for a failure that happened after it was applied.

The numbers below explain the threshold, the condensation mechanism and the desiccant sizing. The coating family is indexed in the surface treatment hub, and the pretreatment sequence that precedes dry-off is set out in seven-tank pretreatment process explained.

What does high humidity storage do to steel?

It puts a film of water on the surface. Steel in clean air at 50% relative humidity carries a few molecular layers of adsorbed water, which is too thin to support corrosion. As relative humidity climbs past about 60%, the film thickens to tens of layers, ions move through it, and rust appears at a rate that rises steeply with every further percent. The threshold is not a law of physics; it depends on surface contamination, but 60% is the working number and 50% is the safe target.

Temperature decides whether the film forms at all. Air at 30 °C and 80% RH holds about 24.3 g of water per cubic metre, and its dew point is close to 26 °C. Any surface at 25 °C in that air is below the dew point and collects water, even though the air is not saturated. That is why a cold steel stack brought into a warm shed drips, and why the underside of a roof panel is wet at dawn.

The daily cycle does the rest. A warehouse warms by 10 K during the day and cools at night. Air leaks in through doors and eaves, carrying moisture; the steel cools faster than the air because it radiates to the sky through the roof, and the moisture condenses on it. By morning the part is wet, and by afternoon it is dry with a ring of corrosion product where the water pooled.

Where does the water come from?

The first source is the air itself, and it is the one nobody can see. A 1,000 m³ stores area at 30 °C and 80% RH holds about 24 kg of water vapour. Warming that air to 35 °C drops the relative humidity without removing any water, which is why a hot afternoon feels dry and a cool night brings the moisture back.

The second source is the floor and the pallets. Concrete slabs wick moisture from the ground, and a steel part standing on a damp pallet rusts at the contact line first. A 20 mm gap under the pallet, or a plastic slip sheet, breaks the wick.

The third source is packaging. Corrugated cardboard sits at 6-10% moisture by mass and gives it up slowly inside a closed carton. Wooden crates are worse, because fresh timber can be above 15%. A sealed crate with 20 kg of timber and no barrier is a humid chamber with a steel part in it.

The fourth source is the process. Parts that leave a rinse tank with water in a hem, a tube or a folded seam carry that water into storage. A dry-off oven removes surface water but not water trapped in a closed section, so the design has to give the water a way out. Drain holes and open hem edges are storage features as much as they are coating features.

The fifth source is the part's own temperature. A part blasted or coated in a cool morning and moved into a warm, humid bay will condense. The rule used before blasting applies after it too: keep the surface at least 3 °C above the dew point until the part is dry and packed.

Storage conditions against the risk they create

The table pairs a storage condition with the mechanism it triggers and the control that actually works.

Why is the gap between pretreatment and coating the riskiest?

A conversion coating is not a barrier. A zinc phosphate coating weighs 1.5-4 g per square metre and is crystalline and porous; its job is to give the paint a key and to slow the spread of corrosion under a damaged film. Until the primer or powder seals it, the phosphate holds moisture against the steel in exactly the places where the crystals are thickest.

That makes the hours between the dry-off oven and the coating booth the highest-risk period in the route. A part that waits overnight in a humid bay, or over a weekend, will show rust bloom at the phosphate grain boundaries, and the powder applied over it will blister within a season. The fix is scheduling, not chemistry: pretreat and coat in the same shift where possible, and where a part must wait, keep it in a dry, warm holding area rather than an open rack.

Bare laser-cut parts have their own problem. The cut edge carries a thin oxide and a rougher surface than the rolled face, and it holds moisture longer. On a 3 mm CRCA bracket, the edge is a small fraction of the area but the first place rust appears, which is why a rust-preventive oil or a quick phosphate is worth more on a laser-cut part than on a sheared one.

The same logic applies to parts that are coated on one side only. The uncoated face is a reservoir: it rusts, the corrosion product swells, and it pushes the film off from behind. If one side must stay bare for welding or earthing, say so on the drawing and protect that face with oil until it is used.

How much desiccant does a crate need?

Size the desiccant for the water in the air, not for the volume on the label. A crate with 2 m³ of free air at 30 °C and 80% RH holds 2 x 24.3 = 48.6 g of water vapour. Moving that air to 50% RH leaves 2 x 15.2 = 30.4 g, so 18.2 g must be absorbed.

Silica gel takes up roughly 20-25% of its own mass at 80% RH, so 18.2 g of water needs about 73-91 g of gel. Add the moisture that comes out of a plywood crate and a cardboard carton, and the practical figure is 100-150 g for that crate, in a breathable sachet placed at the top where the water vapour collects.

Two rules make the calculation hold. The bag must be a barrier, because a woven sack or a ventilated crate exchanges air with the room and the desiccant is exhausted in a day. And the part must go in cool and dry, because the calculation assumes the air inside the bag is the air you measured, not air that condensed on the steel during packing.

A humidity indicator card costs less than the desiccant and tells the store whether the pack is still working. Read it at goods-in, not at the assembly line.

VCI is not a substitute for desiccant, and the two are often confused in a purchase order. A vapour-phase corrosion inhibitor saturates the air inside a closed pack with an amine that passivates the steel surface, so it stops corrosion from starting on a dry part. It does not remove water, and it does not stop condensation on a cold surface. Use VCI where the part is clean and dry and the pack is closed, and use desiccant where the pack contains humid air or a moisture source such as timber. A pack that needs both is normal for a long storage period; a pack that uses VCI to compensate for a wet part is a pack that will fail.

One measurement is worth more than any of this arithmetic. Put a data logger in the stores for a week and record relative humidity and temperature every 15 minutes. A store that averages 55% RH but touches 85% at 4 a.m. is a condensing store, and the average hides the event that causes the rust. The logger also tells you whether the problem is the building or the pack, which is the first question a coating supplier will ask.

What Ganesh runs for parts that have to arrive clean?

Ganesh pretreats through the seven-tank line, dries off before coating, and powders in an 18 ft x 14 ft booth or wet-paints in the wet booth, with a curing oven completing the film. DFT is measured with a gauge and adhesion with a cross-hatch test, and the results are recorded against the batch so a storage query can be traced to a coating history rather than to a photograph.

The plant packs and dispatches from the same units, so the gap between the last process and the crate is measured in hours, not days. Where a drawing calls for barrier bags, desiccant or a rust-preventive oil on machined faces, that is agreed at quotation and written into the packing instruction; the paint shop capability sets out the line sequence.

The five units across Mumbai (Vasai) and Nagpur hold ISO 9001:2015 and ISO 14001:2015, and the effluent treatment plant handles the pretreatment waste stream. What the plant cannot control is the humidity of the buyer's stores, the temperature of the truck, or how long a crate sits on a dock. Those are specified by the buyer and, in practice, are where most storage rust starts.

What belongs on the drawing and the purchase order?

1. Coating system and DFT, with the faces to be coated and masked.
2. The maximum time between pretreatment and coating, where the buyer is setting the route.
3. Rust-preventive oil on machined or uncoated faces, with the film weight in g per square metre.
4. Packing method: open rack, barrier bag, or sealed crate, and who supplies the packaging.
5. Desiccant quantity and placement, and whether a humidity indicator card is required.
6. Handling: gloves for phosphated parts, no cardboard contact with coated faces.
7. Storage climate at the destination, in relative humidity and temperature range, so the packing can be sized for it.
8. Shelf life and the re-inspection interval before a stored batch is released to the line.

When is coating the wrong protection?

A coating protects a surface it can reach. It cannot protect the inside of a closed tube, the faying surface of a bolted joint, or a machined bore that has to stay within a few micrometres. Those need a different answer: a drain and a sealed end for the tube, a sealant or a plated fastener for the joint, and a rust-preventive oil or a VCI emitter for the bore.

Parts that will be welded after coating also need thought, because welding burns the film back and leaves a bare zone that rusts in storage. Either weld before coating, or mask the weld zone and protect it with a weld-through primer agreed with the welding engineer.

For everything else, the sequence is the answer: dry the part, cool it, coat it, pack it cool and dry, and hold the stores below 50% RH. A batch that follows that sequence does not need a thicker film or a more expensive powder. Send the drawing with the storage climate and the packing requirement through the quotation route, and the packing can be sized before the first part is coated rather than after the first complaint.

Related reading: the Coastal site corrosion: design, failure modes and the fix and the Degreasing aluminium parts before coating take the same problem further.