silicon for the chemical and solar industry XIII

Silicon for the Chemical and Solar Industry XIII

Kristiansand, Norway, June 13 – 16, 2016

Identification and root cause analysis of micro-cracks in a

trichlorosilane reactor

Sean Gaal

, Bill Larson

, John Herman

, Eric Davis

1) Dow Corning Corporation, Midland, MI, USA

2) Hemlock Semi Conductor, Hemlock, MI, USA

Abstract

During a routine inspection cracking was identified in a carbon steel trichlorosilane

fluid bed reactor in the weld heat affected zone under a lifting lug. Further

investigation using wet fluorescent magnetic particle inspection revealed more

extensive cracking. A root cause analysis was performed which identified the

mechanism as chloride salt induced stress corrosion cracking. Methods are identified

to mitigate the risk.

Introduction

Stress corrosion cracking (SCC) of metals is a form of environmental cracking in

which cracks form in the simultaneous presence of a tensile stress (as an applied stress

due to live loading or as a residual stress from fabrication) and a corrosive

environment. A unique characteristic of SCC is that the metal dissolution component

of the corrosion reaction is essentially localised to the site and interior of the cracks,

generally this is the primary factor in determining the rate of propagation. This type of

localisation is atypical, it is only found in certain combinations of metals and

corrosive environments. In corrosion engineering, this has led to the compilation of

SCC susceptibility for a given metal by associating it with specific corrosive agents or

environments.

Identification

Trichlorosilane reactors are routinely inspected, as part of the maintenance schedule.

To accomplish this, they are first emptied and then washed out with water,

hydrolysing any residual chlorides. During a routine visual inspection some small

cracks were observed on the inside of the reactor, corresponding to the location of a

lifting lug which had been welded to the vessel exterior. To better characterise the

cracks the inside of the reactor was sand blasted and the cracks were imaged using

wet fluorescent magnetic particle (WFMP) analysis (Figure 1). It was quickly

discovered that the cracks were significantly more extensive than initially realised,

located under the lifting lug near the man way and in several areas of the knuckle

radius.

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