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Failure Analysis of Corroded Rupture Disc

ENVIRONMENT:

Sulfuric Acid

EQUIPMENT:

Process Equipment

MATERIAL:

Alloy C-276

FAILURE:

Corrosion
 

Background

The company manufactures equipment that is used in the processing of silicon wafers. Part of the process involves the use of hot ozonated sulfuric acid. Since the system is under slight pressure, a rupture disc is used to protect the equipment from excessive pressures.  

The rupture disc is constructed of Alloy C-276 with a thin layer of Teflon FEP applied. A skived film of PTFE is placed over the process side of the disc to protect it from the environment and serve as the joint seal. The PTFE film is scored along the outer edge of the rupture disk to facilitate failure of the film upon rupturing of the disc.   

Three exposed discs that had become discolored were submitted for analysis. A failure analysis was requested to determine if the discoloration was due to corrosion and if so, the cause. 

 

FINDINGS 

Visual Examination and Wet Sponge Test

All three ruptured disc assemblies were photographed in the “as-received” condition. The metal discs appeared intact with no visible deformation except for the manufactured dimple at the center of the disk.  

Obvious failure of the PTFE film along the score mark was present on two of the assemblies. These assemblies had light brown deposits underneath the PTFE Film. The PTFE film on the third assembly appeared intact. This assembly had green/blue colored deposits underneath the PTFE film. 

A modified wet sponge test was performed using a digital volt-ohm meter (VOM) and a miniature sponge attached to one lead to determine if any discontinuities existed in the film. The surface of each disk was scanned. No additional film discontinuities were detected on the two disks with the failures along the score marks. However, several small indications were detected along the sealing surface of the third disc. Macroscopic examination revealed the presence of several small holidays in the PTFE film in the areas indicated by the wet sponge test, Figure 1. There was no indication that the underlying base metal had experienced corrosion in this area.

 

 

Figure 1. Discontinuity in PTFE film indicated by wet sponge test, 25X magnification.

 

The PTFE film was loosely adherent to the assembly on all of the discs. The PTFE film was peeled up on the two discs where the film had failed at the score marks. The wet sponge test was performed on the FEP coated C-276 disc. The test indicated that the coating was discontinuous and did not provide any significant protection of the underlying metal. 

The light brown deposits present on the discs where the PTFE film had failed at the score marks were loosely adherent and cleaned off easily. There was no definitive indication that corrosion had occurred underneath these deposits.

Chemical Analysis

The PTFE film on the disc with the green/blue deposits was carefully removed to examine the underlying metal and deposits, Figure 2. The deposits on the metal disc were lifted off with conductive double stick tape and placed onto a carbon stub. The stub was inserted into the goniometer stage of a scanning electron microscope (SEM) for elemental analysis of the deposits using the energy dispersive x-ray spectrographic (EDS) analysis technique.  

 

 

Figure 2.  Rupture disc with PTFE film removed

 

The EDS spectra indicated the composition to be primarily sulfur (27.5%) and calcium (24.3%) with major peaks for nickel (18.5%), iron (12.1%), and chromium (11.9%) and minor peaks for silicon (4.3%) and aluminum (1.4%). The presence of sulfur is most likely due to sulfuric acid. The source of the calcium is unknown unless it is a constituent of the adhesive used to hold the PTFE film in place. The nickel, iron, and chromium are most likely due to corrosion of the base metal although the iron is higher that expected. The presence of aluminum and silicon in corrosion deposits are usually associated with particulate matter.  

A small section of the metal disc was removed, polished and placed into the SEM to confirm the elemental composition of the metal. EDS analysis indicated that the composition of the disc was within the chemical composition of Alloy C-276. 

 

Discussion

The above analysis has indicated several potential problems that may lead to premature failure of the disc due to corrosion. They are: 

·       The FEP lining on the discs are discontinuous and of no practical value for corrosion resistance.

·    The score mark on the PTFE film was fragile leading to the mechanical failure of the film prior to rupturing of the metallic disk.

·        The PTFE film is being permeated by the ozonated sulfuric acid.

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