Case Study - Polyvinylidene Flouride Chlorine

History


Figure 1 Cross-section of a PVDF pipe socket joint

PVDF combines melt processability with the excellent chemical resistance expected of a fluoropolymer. It is readily available in the form of extruded sheet & pipe that can be fabricated by thermal welding techniques. These attributes have made the material very attractive for the construction of process plant for handling corrosive fluids.
In the late 1970s, PVDF extruded pipe of ~ 150 mm diameter & several kilometres in length was chosen to transport dry chlorine gas to various reaction vessels within a large industrial complex. Pressures & temperatures were modest & the pipe was deemed to be structurally adequate without the addition of external reinforcement (e.g. GRP lamination).
Spigot/socket joints were employed as shown in Figure 1. The sockets were fabricated by welding.
Within eight weeks the pipe system developed several hundred serious leaks at cracks within the socket collars.

Inspection & Analysis

On inspection it was established that all collar cracks had initiated at, or adjacent to, the internal collar weld; that is where the collar is exposed to the chlorine gas. Photographs are shown in Figure 1 & 2.
The circumferential cracks tended to initiate in the upper quartile (the crown) of the pipe cross-section.
Significant swelling of both the collar & the pipe end was observed.
Electron probe microanalysis of the pipe wall of cracked samples revealed chlorination of the PVDF (hydrogen abstraction/chlorine substitution).


Figure 2 External view of a typical leakage crack

Failure Diagnosis

The resistance of PVDF to dry chlorine gas is described as excellent in the supplier's data sheets, yet it was evident that very rapid attack had occurred in this case. The very localised nature of this attack initially suggested that the heat affected zone at or near to the weld may have been rendered prone to chemical attack.
However, it was noted that the degree of attack was most severe in pipe sections that were exposed to outdoor conditions. This combined with upper quartile initiation suggested that sunlight was involved.

It was established that the translucent PVDF pipe was sufficiently transparent to UV to dissociate molecular chlorine:
Cl2 Cl• + Cl•

The concentration of highly reactive chlorine radicals maximised in gaps between the pipe ends where gas flow was at a minimum. Hydrogen abstraction & chlorine substitution caused swelling & embrittlement, & cracking at the points of maximum stress concentration (the internal socket weld). As the cracks developed the concentration of chlorine radicals increased at the crack tip (increased UV intensity + reduced gas flow rate). Hence very high rates of corrosion stress cracking were seen.

Lessons & Consequences


Figure 3 Cross-section of a typical leakage crack
  1. PVDF is severely attacked by atomic (singlet) chlorine radicals.
  2. Chlorine radicals can be generated by exposing molecular chlorine to sunlight or artificial lighting with significant radiation in the 300 - 400 nm range.
  3. Transparent or translucent polymers should not be used to transport or contain fluids that are prone to dissociation by UV.
  4. The installation was replaced with pigmented PVDF & subsequently this proved to be successful.
  5. The material supplier gained valuable knowledge from this expensive failure & agreed to bear the cost of the replacement pipe.
  6. This was the first of many similar failures involving PVDF & ECTFE with chlorine & chlorinated chemicals (e.g. sodium chlorate). Only three references to this important phenomenon [5, 22, 23], were found in the literature. This highlights the need for very exhaustive searches of public domain literature to minimise the risk of repeated historical failures.

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