STRESS CORROSION CRACKING

Stress Corrosion Cracking

Table of Contents

Chapster One: Introduction4

Introduction4

Stress Corrosion Cracking5

SCC6

Chapter Two: Literature Review7

What causes stress corrosion cracking?7

Active path dissolution7

Hydrogen embrittlement8

When will SCC occur?9

Location of SCC Incidents10

Brass in ammonia-containing environments10

Chloride cracking of stainless steel11

Pipe and Coating Characteristics Associated with SCC Incidents11

Pipeline Age at SCC Occurrence13

Soil Conditions14

Cracking Environments14

Role of Stress16

Conditions for SCC16

The Incidence of SCC C Summary of Controlling Factors18

Field Digs and Monitoring19

High-pH SCC Protocol and Field Monitoring19

Field Digs20

Data Analysis and Decisions21

Pressing Sheet Metal28

Yield point elongation28

Anisotropy28

Residual Stresses28

Spring-back29

Wrinkling29

Casting30

Rolling31

Grain Boundaries when you roll Steel32

Forming Extrusion33

Ford's Press Tooling Requirements for Tryout operations35

Punches & Dies37

Relationships of Piece Part sizes to Punch and Die sizes38

Re-usable and non-permanent moulds & Suitable casting materials for particular processes38

Sand casting39

Die casting40

Gravity die-casting40

High-pressure die-casting40

Low-pressure die-casting41

Vacuum die-casting42

Squeeze casting (or squeeze forming)42

Conclusions43

References45

Chapster One: Introduction

Introduction

This paper addresses external stress-corrosion cracking (SCC) on gas transmission pipelines. SCC was first recognized during the course of analyzing a gas-transmission pipeline rupture that occurred in Louisiana in 1965. However, comparing the metallographic and fractographic features, that have since become synonymous with SCC with similar information archived in pipeline failure analysis reports at Battelle indicates that the first SCC failure probably occurred in 1957.

This paper begins with a detailed review of the incidence and character of SCC. That review is followed by topical sections titled Controlling Factors, Field Digs and Monitoring, and Management and Mitigation. While SCC has been a problem in transmission pipelines since the 1960's and much has been written on each of these topics, significant work remains to be done before the cause of SCC is determined and practical mitigative schemes are developed. Thus, this paper discusses these topics briefly and in rather generic terms. How this discussion applies to any given pipeline is very case-specific C because of the large number of factors that control the susceptibility to SCC and its kinetics. The paper closes with conclusions drawn from the discussion of these topics.

Stress Corrosion Cracking

Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses.

Despite the introduction of polymers and composites in recent years, metals remain important in structures because of their strength, stiffness, toughness and tolerance of high temperatures.

Unfortunately, metals are subject to corrosion. (The noble metals, such as gold and platinum are an exception to this, but they are rather too rare for common use). Corrosion can take many forms; the form that concerns us here is the interaction of corrosion and mechanical stress to produce a failure by cracking. This type of failure is known as stress corrosion cracking, often abbreviated to SCC.2 As will be explained below, SCC may occur by a number of mechanisms; when cracking is clearly a result of hydrogen embrittlement, this term may be used in place of SCC.

However, this distinction is rather arbitrary; we are often unsure of the mechanisms of SCC, and many failures that are actually due to the effects of hydrogen would conventionally be ascribed to SCC. Similarly other specific stress corrosion cracking processes have acquired their own names; ...