Guides / Materials
Spray rigs eat aluminium, so which parts should be stainless
September 6, 2026
An agricultural aircraft is the hardest environment a commercial UAV works in. It carries a tank of something corrosive, it sprays that something into its own downwash, and it gets pressure washed at the end of the day. For anodise, chemical film and powder coat on the aluminium parts, see our guide to drone parts finishing.
Parts that last five years on an inspection aircraft last one season here.
What is actually attacking the metal
Three different mechanisms, and they need different answers.
- Chemical attack. Many liquid fertilisers are alkaline, and aluminium oxide dissolves in alkali. That is the layer keeping the metal alive, so once it goes the metal goes. Chloride based products pit both aluminium and 304 stainless.
- Galvanic corrosion. Carbon booms against aluminium clamps, wetted constantly by a conductive liquid, is the fastest version of the problem covered in aluminium and carbon fibre joints.
- Erosion and cleaning. Pressure washing strips coatings and drives water into joints. Sand and grit in the downwash sandblast leading faces.
A part by part answer
| Part | Recommendation | Why |
|---|---|---|
| Nozzle bodies and spray bar fittings | 316 stainless, or POM | Permanently wetted with the chemical |
| Pump mounts and tank brackets | 6061 with hard anodise, sealed | Splash, not immersion |
| Tank hardware, clamps and straps | 316 stainless | Wetted during filling and leaks |
| Boom and arm clamps | 7075 hard anodised, with an insulating sleeve | Structural, weight critical, wetted |
| Motor mounts | 6061 hard anodised | Above the spray, but in the downwash |
| Landing gear feet | PA66 or 7075 hard anodised | Impact plus mud and chemicals |
| Fasteners in wetted areas | 316 stainless, isolated from aluminium | Cheap to upgrade, expensive to replace in the field |
| Sensor and flow meter housings | 6061 hard anodised, or POM if the chemical touches it | Depends on whether it is wetted |
The pattern is simple. Anything permanently in contact with the chemical goes stainless or plastic. Anything structural stays aluminium and gets a proper finish, because tripling the weight of a clamp is not an option.
Why 316 rather than 304
The two look identical and cost almost the same on a small machined part. 316 contains molybdenum, which makes it markedly better against chlorides and against pitting in general. On a nozzle body or a tank fitting that difference is the whole point.
Use 304 where you want stainless for wear or strength in a dry area, and 316 anywhere the product touches. Specify the grade on the drawing rather than writing stainless, because a shop buying to a drawing that says stainless will buy the cheaper one, correctly.
Both need passivation after machining. Machining smears iron across the surface and leaves it free to rust, which is why a new stainless part sometimes shows orange spots in a week. Passivation to ASTM A967 removes it. Put it on the drawing.
The finish decisions that actually matter on aluminium
For the aluminium parts that stay aluminium:
- Hard anodise, type III, sealed. Thicker and more resistant than type II, and it survives cleaning. The thickness changes fits, so read anodising UAV parts before you dimension the bore.
- Seal the joints. Wet assembly with a sealant on clamp faces keeps the chemical out of the gap where it would otherwise sit.
- No bare machined faces after coating. A face machined after anodising is an unprotected window. If a feature has to be machined last, say what protects it afterwards.
- Drain paths. A pocket that holds liquid will corrode from the inside. A 3 mm drain hole in the low corner costs nothing and saves the part.
Design details that pay for themselves in one season
- Avoid blind threads pointing up. They fill with liquid and never dry. Through holes drain.
- Radius the corners where spray runs. Sharp internal corners hold residue.
- Keep dissimilar metals apart. Stainless fastener into an aluminium boss in a wet area needs an insulating washer, a sealant or a stainless insert.
- Make the wear parts replaceable. A nozzle body that unscrews from a bracket is a five minute job. One machined into the bracket means replacing the bracket.
- Mark the parts. Laser marked part number and revision survives the season and tells you which version failed. It is the cheapest reliability data you will ever collect.
What to send us
The chemical the aircraft actually sprays, or at least whether it is acidic, alkaline or chloride bearing. That single line changes the material and finish recommendation on half the parts in the list above, and it is free to include.
Send the model and that note through the RFQ page and the quote comes back within 24 business hours, with any part we think is in the wrong material flagged before you commit a revision.
See also: corrosion tests for agricultural drone parts for salt spray and chemical exposure tests.
Frequently asked questions
Which is worse for aluminium, salt air or fertiliser spray?
Fertiliser, usually. Many liquid fertilisers and some pesticide carriers are alkaline or contain chlorides, and both attack aluminium oxide directly. Coastal salt air is a slow problem, a spray tank leak is a fast one.
Is 316 worth it over 304?
For anything the chemical touches, yes. The molybdenum in 316 makes a real difference against chlorides and it costs a fraction more on a small machined part. Use 304 for dry structural parts where you want stainless for wear rather than chemistry.
How much weight does stainless add?
Roughly three times the density of aluminium, 8.0 against 2.7 grams per cubic centimetre. On a 20 gram fitting that is 40 extra grams. That is why stainless goes on small wetted parts and never on the airframe.
Need this part quoted?
Send the model and the quantity. Price and lead time come back within 24 business hours.