Case Study - Leaking Acetal Pipe Fittings

Smithers Rapra were approached to investigate the failing in use of an Acetal plumbing fitting that caused damage to the value of £100,000

History

It has become a common practice to construct plumbing systems using a combination of copper pipe, soldered copper fittings, & acetal compression fittings. The latter are often included at locations where it is anticipated that the pipework will require periodic disassembly.

In this case an acetal fitting failed on the 5th floor of a recently built & as yet only partly occupied, office block. The exact time of the failure was not known but could have been up to 72 hours before discovery. During that time up to 25,000 gallons of water would have been released. The cost of damage to the lower floors was estimated to be in excess of £100,000. The insurers agreed to the liability but made it quite plain that they would not renew until the risk of a repetition was well & truly established.

Inspection & Analysis

A picture of the failed fitting is shown. It shows failure based on severe corrosion & embrittlement.

On the upstream side of the fitting is a soldered copper bend. A viscous deposit was found on the bore surface of the bend. Analysis revealed a high level of zinc chloride in the deposit. Our Consultants were immediately aware that this was the probable source of the problem but had no idea as to the source of the aggressive deposit. A passer-by remarked that the deposit looked like a run of plumbers' flux. Conventional plumbers' flux was purchased & found to contain a high level of zinc chloride.

Failure Diagnosis

Zinc ions catalyse the initiation of an unzipping (depolymerisation) reaction in acetals. Once initiated the reaction is strongly autocatalytic if the reaction byproducts (e.g. formic acid) are not removed.

In this case the fitting was exposed to a concentrated solution of zinc ions for about 2 years before failure. During this time the system contained static water.

The kinetics of the interaction are particularly complex. The catalytic byproducts are highly mobile so that near surface degradation is retarded. Rapid degradation will occur in the bulk of the material but only after the long delay time required for deep diffusion of the zinc. Long-term immersion testing was required to prove the effect.

It is of interest to note that following this investigation, the failed fitting was stored in a sealed plastic bag. Some years later the contents were needed for re-examination but 90% of the sample had disappeared. The solid acetal, imprisoned in an atmosphere of its own degradation products, had depolymerised to the point of gasification.

Lessons & Consequences

1. Unlike the incompatibility between zinc chloride & Nylons, the incompatibility with acetals is not well known. Many material suppliers claim compatibility. A search of the literature published over the last 10 years reveals no reports of failure & no relevant phenomenological research.
2. Conventional plumbers' flux containing zinc chloride should not be employed in installations that include acetal fittings.
3. The only reason why this type of failure is relatively rare is because in most cases the pipework is well flushed soon after installation.
4. All the acetal fittings were replaced within the building & the system was thoroughly flushed. Reinsurance was granted.