Page 513 - Corrosion Engineering Principles and Practice

480 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 481

Findings
(a) In contact with a liquid phase
i. At point of high flow
• Impingement of solids
• Formation and collapse of bubbles
ii. At point of low flow
• Under debris
Procedural step • Associated with organic deposits
iii. In a crevice
I - Failure is in wall of tube or vessel
iv. At point where temperature varies sharply
(a) In contact with a liquid phase • High negative heat transfer
(b) Related to surface of liquid • High positive heat transfer
∗ Formation of pits under debris
• Near liquid/gas interface ∗ Brittle fracture and hydrogen “Fish Eye”
• Parallel to surface ∗ Thinning without deformation
(c) In Gas or vapor ∗ Thinning with bulging
v. Related to junction between dissimilar metals
(d) Not related to the geometry of tube or vessel
vi. Related to preexisting flaw or segregate
II - Failure is at mechanical joint vii. Related to a weld
• In filler metal
∗ Corrosion
∗ Yielding
Findings • In heat adjacent zone (HAZ)
viii. At locations of high stress
II - Failure is at mechanical joint ix. Horizontal grooving related to stratification
i. Gasket or seal has failed
ii. Faces of joint have separated Findings
• Due to corrosion (c) In gas or vapor
• Due to strain i. At a point of high flow downstream of a barrier
∗ Caused by temperature and pressure ii. General corrosion at point of high temperature
∗ Caused by stresses
iii. Intergranular penetration
iii. Bad fitting
FIGURE 12.2 Recommendations for relating the origin(s) of failure to plant
geometry.

The more detailed an analysis is, the better the events and
mechanisms that contribute as the roots of the problem can be
elucidated. The analyses can be further divided into three categories
in order of complexity and depth of investigation:
Component failure analysis (CFA): It looks at the piece of the machine
that failed, such as a bearing or a gear to determine the specific
cause of the failure (fatigue, overload, or corrosion) and that there
were these x, y, and z influences.
Root cause investigation (RCI): It is conducted in much greater depth
than the CFA and goes substantially beyond the physical root of a
problem to identify the human errors involved. It stops at the major
human causes and does not involve management system deficien-
cies. RCIs are generally confined to a single operating unit.
Root cause analysis (RCA): It includes everything the RCI covers
plus the minor human error causes and, more importantly, the
management system problems that allow the human errors and
other system weaknesses to exist. An RCA can sometimes extend
to sites other than the one involved in the original problem.