Selecting high-capacity anodizing tanks

Q: We need high-capacity anodize tanks for a busy shop. How can we make sure they’re made right, won’t leak and won’t cause a disaster if someone backs a fork truck into one?

A: Process tanks subject to high temperatures and corrosive compounds, in factories with elevated potential for mechanical damage, need careful qualification to be successful long-term — and by “long-term,” I mean decades. The expected service life of a properly qualified anodize tank, used continuously or nearly so, is north of 20 years.

To achieve that longevity, chemical compatibility, operating temperatures (and peak temperatures) and durability are the buyer’s first priorities. Durability, of course, includes several factors related to structural design, including compressive stress potential and the specific gravity (relative density) of the tank contents. Thermal expansion (linear and volumetric) is also an important design factor. 

Given those considerations, the four most effective material options are homogeneous thermoplastics, including polypropylene, co-poly and CPVC*, and steel alloys. Coated mild steel and lined carbon steel are workable for lighter duty applications, but generally don’t have the staying power required for commercial anodizing.   

Homogeneous thermoplastics are favored because the material has a high strength-to-weight ratio, is corrosion-resistant and highly resilient. Tanks are easily repaired if there is a wayward forklift; properly patched thermoplastic tanks maintain their integrity throughout their service life. They’re also easy to install in the field.

The highest quality thermoplastic tanks are fabricated from virgin material. That material, however, has a requirement of its own for best results: fabrication machinery and tooling that can perform the secondary fabrication processes required while maintaining the structural integrity of the material. 

Extrusion welders, butt welders and CNC machines are the three most necessary tools for fabricating sheeted thermoplastics destined for heavy service. Extrusion welding is a continuous, rather than “stop-start” process that minimizes the risk of air pockets forming in the material and ruining a weld. It also eliminates the need for workers to scrape off the contaminants that can compromise weld quality.

Butt welds provide total fusion of the filler material and are generally stronger than the materials they’re joining. Butt welds are the permanent leak preventive for thermoplastic tanks; they also minimize distortion.

In the hands of experienced design engineers, CNC machines and software produce tanks with the closest tolerances and most precise finishes. 

There are other tools and processes needed to fabricate tanks from less-common thermoplastics, including CPVC*; we can explore these separately.

As sales manager for Tri-Mer Corp., I’ve had numerous “emergency” visits to finishing facilities where tanks — often not very old — were weeping chromic acid, leaking at the bends or showing other chemical damage. It was usually easy to see that the tools used to make those tanks were not up to the task demanded by an industry where solvents, acids, alkalis and heavy metal solutions are commonplace. The direct costs of draining and decontamination — and downtime — caused by these occurrences were always consequential.

One side note about tank damage: on several occasions, the cause of those tank emergencies was not obvious — and the list of possibilities was as long as the number of fingers pointing. 

Was it valve vent failure? Thermostat malfunction? Overheating of fluids that precipitated a vapor release? When the cause is unclear, it pays to have partners. Our longstanding relationship with the manufacturer of our thermoplastic sheet material enabled us to quickly arrange for a detailed assessment by that manufacturer’s lab. That association, and our ability to guarantee there would not be a repeat of the issue with the new tank we built, established a 3-way relationship that all the affected shop owners found valuable.

Process tanks are literally the foundation of finishing. There’s no substitute for thoughtful design and selecting a vendor based on their experience, fabrication resources and others they can add to your team — whether the RFQ is for a single pretreat tank or a 14-stage anodize line.

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*CPVC - Chlorinated Polyvinyl Chloride - is a thermoplastic used with corrosive fluids at temperatures up to 210°F.  It has a low affinity to scaling and chemical resistance equivalent to standard PVC.