AMMTIAC Quarterly, Vol. 2, No. 2 - Advanced Materials ...

George A. Matzkanin
H. Thomas Yolken
AMMTIAC
Austin, TX
INTRODUCTION
Corrosion of metallic structures is an industry and governmentwide
maintenance problem that has been rapidly spreading due to
the increased amount of infrastructure and military assets that are
aging. However, even in the case of newer systems and components,
corrosion can be a significant problem because of the harsh operational
environments encountered. Recognition of the severity and
the resulting economic impact of the corrosion problem by various
industries and government agencies has led to significant effort over
the past 50 years to prevent and control corrosion. Nondestructive
evaluation (NDE) plays an important role in this effort, mostly by
enabling the detection of early signs of corrosion so that corrective
action can be taken before the damage becomes severe.
Hidden Corrosion
Hidden corrosion is a type of electro-chemical material degradation
that is not readily or directly detectable visually, or by
any other surface measurement technique.[1] It can often be
detected and quantified in terms of reduction of wall thickness or
structural discontinuities such as pits, flaws and
voids. When attempting to detect material
degradation due to electro-chemical processes,
the corrosion products (e.g., iron oxides,
aluminum oxides, etc.) must be identified so
that an appropriate energy source can be selected
for detection.
In order to perform an inspection for hidden
corrosion, the detection energy source must be
capable of penetrating the material in which the
corrosion is hidden.[1] If the appropriate
source is selected, then the returned signal will
contain an evaluation of the entire material,
including the physical geometry of the component
or system, which may indicate its structural
integrity, and any hidden corrosion. Thus,
the inherent technical challenges are to select
the most appropriate interrogation energy
source and to recover the signal that identifies
the existence of corrosion. Recovering the
desired corrosion data is a mathematical inversion
problem. Depending on the energy source
used, the characteristics of materials, and the corrosion hidden in
structural systems, an exact solution of the inversion problem may
not be feasible. Therefore, data analysis and information processing,
such as the use of neuro-nets, have become key enablers in
developing NDE techniques for hidden corrosion.
The military is considered the primary driver for the development
of corrosion detection technology, while the nuclear, chemical,
petroleum, and oil and gas pipeline industries are secondary
drivers of this technology. This is due in part to the fact that military
systems are typically fielded longer, have higher operational
cycle rates and operate in more corrosive environments than commercial
systems.[2] Aging DoD assets have exacerbated the problem
Figure 1. An F/A-18C Hornet is Moved
to the Flight Deck on an Aircraft Elevator.
(Photo taken by Photographer’s
Mate 3rd Class Todd Frantom and
Provided Courtesy of the US Navy).
of corrosion and have increased the need for prevention, hidden
corrosion detection, and repair. The corrosion battle extends to
essentially the entire spectrum of DoD systems, including surface
ships, submarines, carrier and land-based aircraft, land vehicles, and
amphibious landing craft. As systems age, corrosion becomes one of
the largest cost drivers in life cycle costs of weapon systems. An
example of this problem is the cables that are used for elevators on
aircraft carriers (Figure 1). These cables are outside the carrier hull
and are exposed to the extremely harsh corrosive environment. Due
to the unavailability of NDE detection technology, these elevator
cables are replaced on a time-based schedule every several years at a
cost of hundreds of thousands of dollars per elevator.
PRIMARY NDE METHODS FOR DETECTING
HIDDEN CORROSION
Guided Ultrasonic Waves
Guided ultrasonic wave NDE offers the potential for a costeffective
methodology for inspection of hidden corrosion in large
and sometimes difficult to access areas, such as
insulated piping. The field of guided waves
has reached some degree of maturity, but unfortunately
the number of practical applications
compared to the number of research papers is
rather small.
Guided waves can be used in three regimes,
depending on inspection distance:
• Short range (