Re: [MW:16825] inspection of stress corrosion cracks

Dear sir

Stainless steel is resistant, but certainly not immune, to corrosion.
In a previous article, we examined the need for inspecting stainless
steel pipes further, including a discussion of conditions which
encourage corrosion. When corrosion does occur in stainless steel,
the corrosion often manifests in a very localized manner rather than
uniform (or "general") corrosion. Two forms of corrosion particularly
troublesome to stainless steel are pitting and stress corrosion
cracking, and both can cause very bad problems. Thus, for stainless
steel piping systems, corrosion is an issue worthy of the attention of
plant managers.

In this article, we overview some of the common methods used to detect
corrosion in stainless steel pipes and tubes. Technological
developments have produced a dizzying array of non-destructive testing
(NDT) used for detecting corrosion, each with its own advantages and
limitations. Accurately detecting corrosion in stainless steel in a
cost effective and practical manner is an incredibly complex problem.
Choosing the most appropriate method(s) for your needs requires
consideration of the particulars of the situation and may warrant the
assistance of experts in corrosion detection. For simplicity sake, we
will focus mostly on austenitic stainless steel (or 300 series), which
is susceptible to problematic forms of corrosion and accounts for the
majority of stainless steel in use.

Detection Considerations for Stainless Steel

To provide some assistance in understanding the underlying principles
of corrosion detection in stainless steel, we begin by identifying
three things that make inspection of stainless steel tubing different
from corrosion detection in other materials.

1. Stainless Steel Magnetic Properties

First, most (but not all) stainless steel does not have the magnetic
properties that ferromagnetic materials like typical carbon steel and
many other iron alloys have. Austenitic stainless steel (or 300
series) makes up the majority of stainless steel production and is not
ferromagnetic. Corrosion detection methods relying upon magnetic
interaction with the piping need not apply with non-ferromagnetic
materials. For example, magnetic particle testing such as the Magnetic
Flux Leakage Technique (MFL) will not work on non-ferromagnetic
materials like austenitic stainless steel.

According to experts1, the following detection methods have
applications for stainless steel: Acoustic Emission, conventional Eddy
Current Testing, Infra-red Thermography, Laser Optics, Penetrate Dye
Testing, Radiographic Testing (X-ray), Visual Inspection, and
Ultrasonic Testing.

2. Detecting Pitting and Cracking

Second, many corrosion detection methods do not detect pitting and
stress corrosion cracking. Both of these problems are difficult to
detect and could potentially cause catastrophic failure. The
unfortunate Silver Bridge collapse was caused by stress corrosion
cracking, for example. Thus, if there is a high cost of failure or
there is likelihood of pitting or cracking, appropriate detection
methods must be employed. According to experts1, methods that will
detect cracking and pitting in stainless steel include: penetrant dye
testing, acoustic emission, ultrasonic testing, and eddy current
testing.

3. Insulation Removal

Third, many corrosion detection methods require direct contact with
the metal. Therefore, if there is insulation, the insulation must be
at least partially removed. However, when pieces of insulation are
removed moisture has an opening to get under the insulation, thereby
actually increasing the likelihood for corrosion under insulation.
Furthermore, we've seen firsthand countless times that plugs cut into
the insulation are frequently not properly replaced and are one of the
top contributors to energy inefficiency in piping systems.

This creates a notable dilemma, as there are few instances where you
can accurately and thoroughly check for corrosion under
insulation(CUI) without cutting into the insulation.

Popular Detection Methods

Having identified unique aspects of detecting corrosion in stainless
steel, we will briefly overview some of the more popular detection
techniques.

Visual Inspection

Visually inspecting for corrosion with your own eyes is the simplest
method of all. If you have only a small amount of pipes or tubes, it
may be the cost-effective approach as well. However, for the large
systems home to most stainless steel tubes and pipes, visual
inspection becomes the least cost-effective approach due to the
enormous amount of labor required. In addition, you can't visually
inspect what your eyes can't see, so if there is insulation, you can't
inspect anything that you do not cut off, making it extremely
difficult to detect non-uniform corrosion such as cracking.
Furthermore, the human eye has proved notoriously inept at detecting
stress corrosion cracks, which can start out incredibly small. Relying
solely on visual inspection is almost always not recommended.

X-Ray

X ray (radiography) can be used for corrosion detection without
requiring insulation removal. One disadvantage is that X-rays produce
radiation, and the precautions that need to be put in place when
conducting X-ray testing may be impractical in many environments.
Another disadvantage is that X-ray does not detect cracking and
pitting; however it can be very useful for detecting other kinds of
defects.

Eddy Current Technique

The Eddy Current Technique (ECT) has likely become the most frequently
recommended corrosion detection method for stainless steel. This
technique uses electromagnetic induction to apply alternating "Eddy"
currents to the pipe or tube. As the electromagnetic field interacts
with the material, the impedance of the coil in the testing probe
changes; the impedance paints the picture of the defect in the tube.

Conventional Eddy Current: There are now a few variations of Eddy
Current methods, but Conventional Eddy Current remains what is most
commonly used on stainless steel. The advantages of conventional Eddy
Current are that it is versatile, fast, and cost-effective. The main
downside of conventional ECT is that the material being tested must be
accessible to the probe; thus, if there is insulation on the pipe or
tube, at least some must be removed.

Pulsed Eddy Current: Pulsed Eddy Current testing is a specialized Eddy
Current method that has the fantastic advantage of the probe not
needing to be in direct contact with the material being tested. Thus,
no insulation needs removed. Pulsed Eddy Current has therefore become
a popular method for detecting corrosion under insulation. However, it
has the drawback of not being known as reliable in detect
ultra-localized corrosion like pitting.

Conclusion:

That was just the tip of the iceberg when it comes to corrosion
detection methods. There truly is a tremendous variety of techniques
and none of them seem to come out as a clear winner for detecting CUI
in stainless steel piping in a full range of scenarios. However,
techniques can be combined to better meet inspection needs.

Due to severity of problems that can be caused by CUI and the
difficulty detecting CUI – especially in stainless steel pipes – it is
all the more important to factor in corrosion and detection methods
before piping and insulation is installed. Consider how
corrosion-prone your conditions are and how costly failure would be
due to corrosion; then draft a detection plan before the system is
complete. Consider your insulation system based on your corrosion
detection and prevention needs. For example, removable insulation can
be the superior choice when frequent inspection and maintenance is
required. It is also recommended to enlist in the advice of corrosion
detection experts when choosing detection methods and making detection
plans.

Regards
GSR



On Wed, Feb 20, 2013 at 9:57 PM, Kandula Raju <rgkandula@gmail.com> wrote:
> Good evening to every one,
>
> We are suspecting chloride stress corrosion cracks under the reinforcement
> pad of a pipe support . Material is stainless steel and cracks will be tight
> , small and randomly oriented across the surface and thickness. can anyone
> suggest us the inspection method of corrosion cracks
>
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