Frequently
Asked Questions - General
1.
What is the acceptable corrosion rate?
A. General
good rule of thumb for evaluating
corrosion rates is as follows:
- <2mpy (0.05
mm/yr) excellent
- <20 mpy
(0.5 mm/yr) good
- 20-50 mpy
(0.5-1.3mm/yr) satisfactory
- >50 mpy
(1.3 mm/yr) unsatisfactory
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2. What
is the UNS classification of alloys? A. This is
the Unified Numbering System for all
materials. This system was established to
be able to specifically identify all
alloys, including proprietary alloys, and
to correlate many internationally used
metal and alloy numbering systems for
uniformity and indexing purposes.
The UNS
designations are administered by the
Society of Automotive Engineers (SAE). back
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3. What
are UNS Sxxxx and UNS Nxxxx numbers?
A. UNS S
classifications designate heat and
corrosion resistant steels, valve steels
and iron-base superalloys, where iron is
in predominance (>50%)
UNS N
classifications designate the nickel and
nickel-base alloys, where nickel is in the
predominance (50%).
However, there
are some iron-base alloys, which are
classified with UNS N numbers. These were
grandfathered designations, which were
originally granted to the nickel base
producers, for alloys such as Incoloy
alloy 800 (Fe-Cr-Ni). These are exceptions
to the system. All future alloy
classifications will allocate future alloy
developments according to the predominant
element in their composition. ba
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4. What
are CPT and CCT?
A. These
are acronyms for Critical Pitting
Temperature and Crevice Corrosion
Temperature. These are the temperatures
above which pitting or crevice corrosion
will occur in aggressive chloride ion
environments. These measures represent
methods by which alloys can be ranked
against one another.
These values are
determined from accelerated corrosion
testing in oxidizing ferric chloride
solutions, conducted in accordance with
ASTM Specification G48. back
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5. What
are ISO Standards?
A. The
International Standards Organization has
attempted to harmonize the various
international and individual standards
into one format, thereby pulling together
the North American, European, and Japanese
standards into one classification. They
have also attempted to establish a unified
format for the designation of metals. back
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6. What
does PRE or PREN mean?
A. PRE
stands for Pitting Resistance Equivalence,
while PREN is the acronym for Pitting
Resistance Equivalence Number. These terms
can be expressed mathematically, based
upon the chemistry of corrosion resistant
alloys. This provides a method of ranking
material against one another, regarding
their pitting resistance, with the highest
number providing the greatest pitting
resistance. This mathematical ranking can
also be reflected by running accelerated
corrosion tests on the same products (ASTM
Specification G48). ba
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7. What
is MIC?
A. MIC
stands for Microbiologically Influenced
Corrosion. This is the attack of metal and
non-metallic substrates, including
stainless steel, by microbes. These might
be anaerobic sulfur-reducing bacteria, or
aerobic acid bacteria. These bacteria may
be found in untreated water streams, used
for cooling equipment or hydrotesting of
vessels. Under low flow or stagnant
conditions, the bacteria can incubate and
form colonies beneath tubercules, which
can result in aggressive
micro-environments that can be corrosive
to the material substrates. ba
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