Anodized aluminum cache: What eloxal can do outdoors
An anodized aluminum cache is light and sturdy. It can withstand rain, frost, and leaves for many years if two things are right: the anodized layer is intact, and the seal and threads keep the water out. The metal itself is rarely the problem. The weak points are scratches, holes, threads, condensation inside, and contact with other metals. Here you will learn how the protective layer is created, what happens when your aluminum cache gets wet, what to look for when buying, and how to care for your cache so that the logbook stays dry.
The essentials in brief
- Anodizing (anodizing) electrochemically transforms the surface of the aluminum into a hard oxide layer. It is not a coating that can flake off, but part of the metal.
- The layer is resistant to neutral substances in the pH 5 to 8 range. Rainwater alone does not harm a well-anodized cache.
- Scratches, holes, slots, and threads are critical, because the layer is thinner or missing entirely there.
- Alkaline cleaners destroy the anodized layer over time. Clean your cache only with clear water or a mild, neutral cleaner.
- Whether the contents stay dry depends on the seal, not the aluminum. A weatherproof logbook is therefore mandatory.
Table of Contents
- What anodized aluminum actually is
- How the layer protects against corrosion and where its limits lie
- What happens when the aluminum cache gets wet
- The typical weak points of the aluminum hide
- What to look for when buying an aluminum cache
- Aluminum, stainless steel, or PETG: Which material for what?
- How to properly care for your aluminum cache
- Our perspective as geocachers
- Camouflage that protects your container
- Frequently Asked Questions
- Sources
What anodized aluminum actually is
Aluminum naturally forms an extremely thin oxide skin in the air. This already provides some protection, but is sensitive to abrasion. Anodizing is used to deliberately enhance this natural protective layer.
Fraunhofer IFAM describes anodizing as an electrochemical process: The workpiece is suspended in an aqueous electrolyte and connected as the anode, i.e., to the positive terminal. This causes a micrometer-thick, usually open-pored oxide layer to grow, which increases the corrosion resistance and wear resistance of the base material.
In the German-speaking world, the result is often called “Eloxal,” short for electrolytically oxidized aluminum. Anodized, anodized, anodically oxidized: For you as a cacher, it always means the same thing.
Why the pores are so important
Fresh from the bath, the oxide layer is full of tiny pores. It is precisely into these pores that the dyes enter during dyeing. This is why aluminum caches are available in black, red, olive green, or blue without any paint being applied.
Then comes the crucial step: sealing. According to a description of the anodizing process, the pores are sealed with demineralized water at 100 °C. This forms aluminum oxide hydrate, which hermetically seals the pores. Only then is the surface properly sealed. A poorly or not sealed layer absorbs dirt and moisture more easily and discolors faster.
Eloxal is not a coating
This is important for geocaches. A coating lies on the metal, and once it is scratched, moisture can creep underneath. The anodized layer, on the other hand, grows out of the aluminum. It does not flake off in pieces. If it is damaged, however, bare aluminum is exposed at that point, and it behaves like any untreated aluminum.
How the layer protects against corrosion and where its limits lie
The oxide layer is chemically inert. According to the Guidelines for the Anodizing of Aluminum, anodized aluminum is resistant to neutral chemical substances with a pH of 5 to 8. With sufficient layer thickness and a clean process, the layer can also withstand an acidic or slightly alkaline environment in the short term.
For your cache, this means:
- Rain, dew, fog: No problem for an intact layer.
- Leaves, soil, moss: Organic material keeps moisture on the cache for a long time. This discolors the surface over time, but usually does not seriously attack it.
- Lime, concrete, mortar: Fresh concrete and mortar are highly alkaline. An aluminum cache placed directly into a freshly grouted wall crevice is not well off there.
- Road salt and spray: In winter, salt-laden water reaches the cache at roads, guardrails, and bridges. Salt solutions are considered aggressive to aluminum, especially where the layer is already damaged. Here, plastic or high-quality stainless steel is the more relaxed choice.
The layer only protects as long as it is intact
Every scratch and dent is a possible entry point. Outdoors, this happens faster than you think: The cache is pulled out of a tree fork, pushed over rocks, or pried out of a crevice with a pocket knife. At such points, localized corrosion can then begin, visible as a whitish, powdery deposit.
The guidelines also indicate that anodized aluminum shows good wear resistance when exposed to the elements, provided it is cleaned properly. How to do this correctly, you will read below.
What happens when the aluminum cache gets wet
In short: little to the metal, potentially a lot to the contents.
An intact, sealed anodized layer handles water well. The aluminum nano does not rust, because rust in the true sense only exists with iron and steel. Aluminum can oxidize, but on a well-anodized surface, this process hardly continues because the surface already consists of oxide.
Moisture becomes critical in three places:
- In the threads: Thread grooves are fine grooves. The layer is thinner there, and it is worn down by frequent screwing on and off. Moisture and dirt collect in the grooves. Over time, the lid can become difficult to turn or even seize up.
- At the seal: A rubber O-ring ages due to UV light, frost, and drying out. If it becomes cracked or flat, water gets in. In practice, this is the most common reason for a wet logbook, not the aluminum.
- Inside: If the cache is opened in damp weather, the next finder encloses warm, moist air. If the container cools down at night, condensation forms. This happens faster in metal containers than in plastic because metal quickly transmits temperature changes.
If the threads and seal are in order, your log strip will remain readable even after a rainy day. If it is wavy or stained, check the O-ring first.
The typical weak points of the aluminum hide
Aluminum containers for geocaches come in many forms: small tubes with screw lids as nanos or micros, keychain capsules, larger tubes for roll logbooks, and aluminum tins with magnets. The weak points are similar in all of them.
Holes, eyelets, and slots
Many tubes have an eyelet or a hole for a ring or a wire rope. The same guidelines state that the inside of holes or slots may have a lower layer thickness than the outside surfaces. The protective effect is therefore weaker there. If drilled after anodizing, the layer is missing entirely at this point.
Exactly there, the metal ring also rubs. If you place an aluminum cache with an eyelet, it is better to hang it from a plastic loop or a sleeved wire rope.
Contact with other metals
If two different metals touch and moisture is added, the less noble metal can be attacked more quickly. Aluminum is usually the loser here, for example compared to steel, stainless steel, or brass. Typical pairings on the cache:
- a steel ring through the aluminum eyelet
- a neodymium magnet sitting directly on the aluminum housing
- a stainless steel screw in an aluminum part
As long as the anodized layer in between is intact, it separates the two metals. If it is worn through, an intermediate layer of plastic or rubber helps. How we solve the issue of rust with magnets is described in the post Stopping rust on the magnet tin: screwed on instead of glued.
Layer thickness
Not all anodizing is the same. The anodizing guidelines recommend a layer thickness of about 20 µm for wet areas and for mechanically stressed components. An outdoor geocache is exactly both: wet and mechanically stressed. Purely decorative anodizing is often significantly thinner. With cheap tubes, you will rarely find any specification for this.
What to look for when buying an aluminum cache
Before you hang an aluminum tube on the fence, it pays to take a critical look. You can check these points directly in your hand:
- Uniform color: Splotchy or streaky surfaces indicate an uneven process.
- Clean threads: The thread turns smoothly and without crunching. Burrs and shavings are a bad sign.
- Seal present: An O-ring made of rubber or silicone belongs in every screw lid. Without a seal, the tube is not waterproof, no matter how good the aluminum is.
- Wall thickness: Very thin-walled tubes dent quickly, and the layer tears at a dent.
- Eyelet screwed in or drilled: A subsequently drilled eyelet is an open point in the anodized layer.
- Replacement O-rings: Pack a few matching O-rings in your maintenance bag right away.
And very important: A log strip made of normal paper becomes soggy from condensation. Use weatherproof log paper, even in the tightest container.
Aluminum, stainless steel, or PETG: Which material for what?
Anodized aluminum is not the only good choice. Depending on the location, a different material plays to its strengths.
| Criterion | Anodized aluminum | Stainless steel (V2A/V4A) | PETG (3D printing) |
|---|---|---|---|
| Weight | very light | heavy | very light |
| Scratch resistance | hard, but scratches expose bare aluminum | very robust | softer, scratches remain superficial |
| Water and rain | no problem with an intact layer | no problem | no problem with a tight seal |
| Road salt | sensitive at damaged points | V4A significantly better suited | insensitive |
| Contact with other metals | contact corrosion possible | less noble partners suffer | no problem |
| Condensation | rather stronger | rather stronger | rather weaker |
| Camouflage and shape | tubes, capsules, tins | screws, bolts, tubes | almost any shape, e.g. shield, post, branch |
With stainless steel, the question of the alloy is worth asking. We explain the differences in the post V2A vs. V4A for outdoors. With 3D-printed hides, the surface determines the tightness. How to make a print permanently tight is shown under Sealing the 3D-printed surface.
Our rule of thumb: aluminum for light nanos and micros in protected locations, stainless steel for locations with heavy mechanical stress, PETG everywhere where camouflage and shape are paramount or salt is involved.
How to properly care for your aluminum cache
As the owner, you are responsible for the condition of your hide. With a few steps during every maintenance, an aluminum cache lasts significantly longer.
- Clean only mildly: The anodizing guidelines advise against alkaline cleaning agents because they destroy the anodized layer with prolonged use. Clear water, a soft cloth, and if necessary, a drop of pH-neutral dish soap are sufficient.
- No dishwasher, no washing machine: The German Federal Environment Agency also describes that contact of aluminum parts with highly alkaline wash fluids causes corrosion damage. That is why corrosion inhibitors are added to detergents. For your cache, this means: hand wash.
- Clean the threads: Brush sand and dirt out of the thread grooves with an old toothbrush.
- Check and care for the O-ring: If it is cracked or flat, replace it. A touch of silicone grease keeps it supple and makes the lid easier to turn.
- Dry the inside: Take out the wet logbook, let the container dry open, put in new log paper.
- Monitor damaged points: White, powdery deposits indicate corrosion on bare aluminum. For small spots, it is often enough to keep an eye on them. Replace heavily attacked containers.
No scouring powder, no steel wool, no oven cleaner. Everything that scratches or is highly alkaline destroys exactly the layer that protects your cache.
Our perspective as geocachers
We like anodized aluminum for what it does well: lightweight, small tubes that fit in one hand. For hides that are meant to be camouflaged, we prefer 3D-printed parts made of PETG or handcrafted wooden hides. This allows us to create shapes that no one would think are a cache, and there is no metal pairing that can corrode. We often combine both: a small, sealed container inside and a camouflage cover around it. This way, the container only needs to be sealed, and the camouflage takes care of the housing.
Our advice: Choose the material based on the location, not the price. And plan for maintenance right away.
— Benedikt
Camouflage that protects your container
You don't want to simply hang your nano on a fence? Then check out our hides. The Nano hide for signposts (Ø 60 mm) is 3D-printed and sits inconspicuously on the post. The Rusty Bolt Cache looks like a rusty bolt and comes with its own nano container. You can find logbooks, spare parts, and other materials in our Geocaching Equipment and Accessories.
Frequently Asked Questions
Does an anodized aluminum cache rust?
No, only iron can rust in the actual sense of the word. Aluminum can corrode, however, especially at scratches, holes, and in the threads, where the anodized layer is thin or missing. This appears as a white, powdery coating rather than reddish-brown rust.
Is an anodized aluminum tube waterproof?
The aluminum itself does not let water through. Whether the container is sealed depends on the lid and the seal. Without an intact O-ring, water will get in. Therefore, check the seal during every maintenance and replace it as soon as it has cracks.
What do I use to clean an aluminum cache?
Use clear water, a soft cloth, and, if necessary, a little pH-neutral dish soap. Alkaline cleaners, abrasives, and dishwashers damage the anodized layer. You can clean the threads with an old toothbrush.
Is anodized aluminum suitable for caches by the road?
Only to a limited extent. Road salt and spray water heavily affect aluminum at damaged areas. For roads, bridges, and guardrails, PETG hides or V4A-grade stainless steel are the more robust choice.
How long does an aluminum cache last outdoors?
There is no fixed number. Durability depends on layer thickness, compaction, location, and maintenance. In a protected location with regular maintenance, a good tube will last a long time, while in a salty, harsh environment, it will last significantly less.
Sources
- Fraunhofer IFAM: Surface Treatment by Anodizing
- Rolec: The Anodizing Process (PDF)
- Gleich: Guidelines for the Anodizing of Aluminium (PDF)
- German Environment Agency: Ingredients of Detergents and Cleaning Agents