Aluminum Workshop: Fabricating aluminum planters

Q: I am bidding on a job to fabricate aluminum plant boxes. They will sit inside 4-foot-tall walls in an office. These rectangular boxes, or troughs, will be 7.5 inches tall, 7.5 in. wide, and 48 in. long, placed end-to-end to form 16-ft. dividers.

Why Aluminum Is a Good Material for Planters

A: Actually, aluminum is a good material for your planters. Of all the 5XXX aluminum alloys, you picked the best one, 5052, for this application. While 304 stainless steel will have a longer life than 5052, it will be three times as heavy and several times more expensive. The 5052 planters should last at least 20 years, in any case.

The reason that 5052 is such a good choice is that the corrosion behavior of 5XXX alloys is best when magnesium is kept to low levels in applications with either an acidic or basic environment. Because soil can be either, 5052 is a good choice. Its magnesium content is only 2.5 percent nominally. While 5XXX alloys with higher magnesium content, such as 5083, are stronger than 5052, 5052 will corrode less in an environment in which it’s constantly exposed to wet soil.

If you want to add years to the life of these planters, check the pH of the soil once or twice a year. If the soil is acidic (with a pH level of less than 7), add powdered limestone to the soil. If the soil is basic (with a pH higher than 7), add copper sulfate, commonly sold under the brand name Miracid, dissolved in water, the closer you keep the pH to 7, the longer the planter will last.

I don’t recommend painting or coating the aluminum. It will last longer if left bare because bare aluminum represents a large surface area that will corrode very slowly but uniformly. If you paint the planters, though, stones and dirt will scratch the coating. These scratches will expose a relatively small aluminum area to the wet soil, and corrosion will proceed fairly rapidly in these areas.

Although you didn’t ask about what kind of fasteners (such as screws) or fittings (such as handles or hinges) to use when aluminum is in contact with the dirt, this topic comes up often. This question is important because when dissimilar metals are placed in a wet environment, either one of them can preferentially corrode. This is called galvanic corrosion.

The best practice is to use aluminum fasteners and fittings. If this is done, there is no practical potential for galvanic corrosion. Again, for the reason stated previously, you should not paint the aluminum.

Part Alternatives for Aluminum Planters

If you can’t find the needed parts made from aluminum, the next best choice is stainless steel. In most environments stainless steel fasteners and fittings can be fastened directly to aluminum alloys with no galvanic corrosion problem, mostly because the stainless steel forms a passive oxide coating that helps insulate the two materials from each other. The only time that the aluminum-stainless combination is a problem is in very acidic soils. Under these conditions, the acid soil can remove the passive oxide coating from the stainless steel. If this happens, the aluminum will corrode preferentially and rapidly.

The least desirable material combination is to use aluminum with carbon steel screws and fittings. Unless the two materials are electrically isolated, the aluminum will corrode preferentially. If you must use carbon steel bolts and fittings, the bolts, washers, nuts, and other parts should be coated to insulate them, and fittings should have a thin plastic or rubber sheet inserted between them and the aluminum. In any case, do not paint the carbon steel or the aluminum. If available, galvanized bolts and fittings should be used instead of plain carbon steel because the zinc coating will corrode before the aluminum.

The last related topic to address is the proper technique to embed an aluminum tube or post in concrete. Such tubes often corrode right where the aluminum goes into the concrete. This happens when the concrete is sloped incorrectly. If you allow water to stay on the concrete around the aluminum, it dissolves a bit of the concrete and creates a highly alkaline environment. This corrodes the aluminum. If you grade the concrete so that water drains away from the aluminum, this won’t happen and the aluminum tube or pipe will last several times longer.