Joint Investigation Results Stress Corrosion Cracking of

Compressed Gas Valves and Fittings

Labor/Industry Overview Briefing 19 September 2011

Charleston, WV

Annotated Summary of Discussion

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Joint Investigation Chronology

Stress Corrosion Cracking is a mechanical-chemical process that results when even a very small pit forms in a metal under stress. The concentrated stress extends a crack from the pit by alternate corrosion and stress failure.

The stress responsible is predominately residual concentrated along the grain boundaries from non-uniform cooling, casting defects and machining.

Separation of Nut Securing Nipple – Jan 2011

Failed Valve Fitting on Right

Top of Failed Valve Nut

Separated Top and Base of Valve Nut

Crack in Valve Body That Had Not Yet Failed

Crack in a Nut That Had Not Yet Failed

Crack in Valve Body That Had Not Yet Failed

Crack in a Fitting That Had Not Yet Failed

Signs of Corrosion Contrast w/ Original Yellow Brass on Valve Nut

Brownish-White Corrosion Residue & Reddish Cooper Deposits

Micrograph of Corrosion Pit Showing Crack Development

Stress Corrosion Cracking is Microscopic in Early Stages

Example of Corrosion on Valve Body

Zinc

Energy Dispersive X-ray Spectrometer Analysis of Corrosion Product

Atomic Structure of Brass – Cooper and Zinc

SEM Image of Zinc Oxide Formation in Brass Matrix

In Dezincification, the zinc in the brass alloy is selectively converted to zinc-oxide creating pits that decrease alloy strength and rending it porous. The cooper is redeposited on the surface of the brass creating an anode allowing two corrosion mechanisms to operate simultaneously – chemical and galvanic.

SEM of Porous Surface Deposits

There are three types of dezincification; intergranular, layer, and

pitting (loss of material in the local area) with the latter being the most serious.

Corrosion Pit Approaching 2/1000 of an Inch

SEM Showing Crack Developing From Corrosion Pit

SEM Showing Corrosion Pit Origin and the Developed Crack

Polished Cross-Section of Intergranular Crack

Corrosion Voids Following the Grain Boundaries

SEM of Intergranular Corrosion in Fitting Not Yet Failed

Microgram of Crack Formation in Fitting Not Yet Failed

Manufacturers required to provide specifications and testing of materials used in their valves and fittings. The specification limit brass alloys to those containing no more than 15% zinc or an other proven dezincification-resist metals.

Solution?

CAG Valve Nipple and Nut Specifications

Examples of Non-Compliant Fittings

1.1

34

”

1.0

34

”

Significance of Pitting Depends on Thickness of Metal

Adhere to Compressed Gas Association specifications.

Solution?

An emergency shelter shall be “… constructed such that it will be protected under normal handling …”, “provide for rapidly establishing and maintaining an internal shelter atmosphere…” and “provide proof of current approval for all items and materials subject to MSHA approval.” (WV Code 56-4-8)

Regulatory Note

“Until breathable air, air-monitoring, and harmful gas removal components approved by MSHA … mine operators may comply with the Federal Mine Safety and Health Act (Mine Act) by: (1) Using breathable air, air-monitoring, and harmful gas removal components that are grandfathered for use until December 31, 2013 …” (MSHA Program Information Bulletin No. P10-21, December

9, 2010)

Regulatory Note

“…a refuge alternative structure that has been approved and is in service or components that have been approved and are in use, but require replacement due to damage, must be replaced with a unit or components that meet the requirements of the final rule.” (Federal Register /Vol. 73, No. 251 /Wednesday, December 31,

2008 /Rules and Regulations, p. 80656)

Regulatory Note

Next Steps

• WV and MSHA to Issue Notices re: Refit

• Identify units require immediate action

• Take corrective action on problems

• Complete Part 7 MSHA approvals

• Manufacture new valves & fittings

• Complete plans for refit

• Refit problems first followed by others

• Complete all by 31 December 2013