KROHNE OPTIMASS Sensors Corrosion guidelines EN - Manual (Page 4)

4 Corrosion Guide Rev 1.05 Jan 2006
Element Typical Specification
C 0.02 0.030 Max
Mn 0.70 2.0 Max
P 0.025 0.030 Max
S 0.001 0.020 Max
Si 0.40 1.0 Max
Cr 22.4 21.0...23.0
Ni 5.8 4.5...6.5
Mo 3.3 2.5...3.5
N 0.16 0.08...0.2
Fe Balance Balance
Specifications for SS318L
Element Typical Specification
Cr 21.6 20.00...22.5
Mo 13.7 12.5...14.5
W 2.9 2.5...3.5
Fe 4.7 2.0...6.0
Co 1.1 2.5 Max
Mn 0.3 0.5 Max
V 0.13 0.35 Max
Ni Balance Balance
Specifications for Hasteloy C22
Please be aware that 318L is a “shorthand” expression
KRONHE have adopted for the material to aid our cus-
tomers’ understanding that the material is a low carbon
stainless steel of a speciifc composition, in the same
way that 316L is used as “shorthand” for UNS S31603.
318L is more expensive to use than 316L, but we need
to consider the technical benefits that it brings. 318L
has a much higher tensile strength compared to 316L.
We use this property in our meter to manage the stress
caused by lengthening of the tube under thermal
expansion. If we used 316L, the tube would deform
well below the +100
0 C maximum we specify for 318L.
The corrosion resistance of 318L is equal to 316L. The
standard surface roughness is similar and it can be pol-
ished to the same fine surface finish of <0.5 um Ra for
hygienic and aseptic applications. 318L is extensively
used worldwide, where customers utilise the higher ten-
sile strength to reduce wall thickness (and so the cost
and weight) of process plant and piping. This means that
there should be no reluctance from our customers in
accepting this material.
KROHNE is the only company to offer a stainless steel
straight tube mass flowmeter.
1.4 NACE Information
National Association of Chemical Engineers (NACE stan-
dard MR0175-2000) is a material requirements standard
relating to the general problems of Sulphide Stress
Cracking (SSC) of metals directed towards sour environ-
ments.
1.5 Galvanic Corrosion with Titanium
This can occur when a titanium tube meter is placed in
contact with other metals, for instance in a steel
pipeline. As titanium is a “noble” metal, the other mate-
rial will normally corrode in preference to the titanium.
In the case of stainless steel pipelines, titanium is very
close in the galvanic table therefore galvanic corrosion
is unlikely to be a problem and no precautions are nec-
essary.
If carbon steel pipelines are used, galvanic corrosion of
the carbon steel may occur with certain acids. This cor-
rosion process causes small amounts of hydrogen gas
to be liberated at the wetted metal-to-metal contact
area. This hydrogen gas may then cause embrittlement
in the titanium. This can be avoided by electrically insu-
lating the Optimass from the pipeline using insulating
gaskets, bolt sleeves and washers.
Of course the use of an alternative tube material e.g.
Hastelloy C-22 will prevent the problem entirely.
1.3.1 What is 318L Stainless steel?
Many customers have asked “what is the 318L stainless
steel that you use in the Optimass 7000 single straight
tube meter, and why do you use this material instead of
316L”? Measuring instruments are more normally manu-
factured with wetted parts from 316L stainless steel,
which is a 100% austenic structure steel, with composi-
tion 18% .Chrome, 8% Nickel, 2.5% Molybdenum, with
the balance Iron.
318L is a 50% austenitic / 50% ferritic structure steel
(typically known as "duplex"), with (typical) composition
22% Chrome, 5% Nickel, 3% Molybdenum, with the bal-
ance Iron.
This material is defined under the following internation-
al codes:
•
UNS S31803
•
ASTM A789, A790, A479, A420
•
DIN 1.4462