UNUSUAL INTERGRANULAR
CORROSION FAILURE OF
ALLOY C276
PRESSURE TRANSDUCER DIAPHRAGM
Michael M. James
Senior Consultant - Materials Engineering
DuPont – Houston TX
2
Overview
•  Chemical Plant in the Texas Gulf Coast
•  Manufacture of Vinyl Acetate Monomer
•  CO2 Removal from the Reaction Loop
•  Potassium Carbonate Solution (30%) Used
for CO2 Removal
•  Vanadium Pentoxide Added as a Corrosion
Inhibitor
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Typical CO2 Removal Process
Stack
Acid Gas
Regenerator
Stripper
Absorber
Reboiler
Feed
Gas
Heat
Rich
Solution
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Lean
Solution
4
Absorber
Stripper
Photograph Of The CO2
Absorber and Stripper
Columns
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Overview of Installation / Condition
• Level in the Absorber Column Monitored Using
Differential Pressure Cell Transmitter
• Transmitter Uses Thin Wall Diaphragm Design
• Type 316L Diaphragm Corroded After 4 Years in
Service (Recurring Problem)
• Replaced With Alloy C276, Plant Had One In Stock
•  Thought To Have Better Corrosion Resistance Than Type
316L Stainless Steel in the Process
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Typical Installation Of Differential
Pressure Cell For Level Control
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Thin Wall
Diaphragm
Style DP Cell
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Construction of Thin Wall Diaphragm Sensor
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Photograph Showing Overall View of Lower DP
Cell Installation
DP Cell
DP Cell
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DP Cell
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Installation Of
DP Cell In
The Absorber
Column
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Upper
Lower
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Photo Showing Lower DP Cell And Diaphragm As
Removed From The Column
Deposits
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Photo Showing
Upper DP Cell
And
Diaphragm From
Redundant
Transmitter As
Removed From
The Column
Deposits
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Photograph of Alloy C276 Diaphragm
Removed from Lower DP Cell
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Observations from Visual Examination of
the Diaphragm
•  Deposits Covered The Area Of The Diaphragm
•  The Deposits Could Be Easily Cleaned Off, Not Very
Adherent
•  Under The Deposits The Surface Of The Diaphragm
Appeared Shiny, With Little Evidence Of Corrosion
•  In The Areas Where No Deposit Build Up Was
Present, The Surface Of The Diaphragm Had A Matte,
Rough Appearance
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SEM Photograph of the Diaphragm Surface
Under the Deposits
General
Corrosion
Intergranular
Corrosion
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Macro Photograph Showing Matted Rough
Surface of the Diaphragm
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SEM Photograph Showing Close Up
of Rough Area of Diaphragm
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SEM Photographs Showing Intergranular and General
Corrosion of the Alloy C276 Diaphragm in Rough,
Matted Areas
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Cross-sectional Photomicrograph of
Diaphragm
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Deposit Analysis
Elt.
Line
Intensity
(c/s)
Error
2-sig
Conc
Units
Elt.
Line
Intensity
(c/s)
Error
2-sig
Conc
Units
Al
Ka
8.96
0.631
0.836
wt.%
Al
Ka
8.23
0.524
0.522
wt.%
Si
Ka
32.50
1.202
2.313
wt.%
Si
Ka
19.29
0.802
0.939
wt.%
K
Ka
311.13
3.719
13.457
wt.%
Cl
Ka
9.41
0.560
0.301
wt.%
V
Ka
89.96
2.000
5.860
wt.%
K
Ka
858.34
5.349
27.448
wt.%
Cr
Ka
49.52
1.483
3.507
wt.%
V
Ka
85.02
1.683
4.512
wt.%
Mn
Ka
17.08
0.871
1.472
wt.%
Cr
Ka
262.03
2.955
15.440
wt.%
Fe
Ka
558.31
4.981
63.246
wt.%
Fe
Ka
435.06
3.808
39.465
wt.%
Ni
Ka
46.45
1.437
9.310
wt.%
Ni
Ka
69.70
1.524
10.326
wt.%
Mo
La
16.40
0.739
1.047
wt.%
100.000
wt.%
100.000
wt.%
Coffee Grounds
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Total
Diaphragm
Total
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Control Valve Between Stripper and
Absorber Columns
Alloy C276 Parts Showing Accelerated Corrosion
In Crevice Packed With Coffee Grounds
End Post
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Shaft (in ball)
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Laboratory Testing
• Unable to Get Process Solution Directly from The Absorber
Column During Operation
•  Sample Ports Plugged with Coffee Grounds
• Used Laboratory Solution Based on Best Analysis of Actual
Process
K2CO3
KHCO3
KBO2
V2O5
H2O (L)
TOTAL (L)
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WT
WT
WT
WT
WT
WT
%
%
%
%
%
%
5.88%
25.67%
3.77%
0.94%
63.74%
100.00%
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Laboratory Testing
•  Corrosion Coupons Installed In Vertical Orientation
•  Alloy C276 (UNS N10276)
•  Alloy 625 (UNS N06625)
•  6- Mo Alloy (UNS N08367)
•  Type 316L sst (UNS S31603
•  Covered Vertical Coupons with Coffee Grounds Removed
from Absorber
•  Second Set of Coupons Suspended in the Solution
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Laboratory Testing – Corrosion Rates
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Alloy Designation
(UNS)
Common Alloy
Name
Exposure
Corrosion Rate
mm/y (mpy)
Corrosion Type / Comments
S31603
316L SST
Solid
0.001 (0.04)
Unattacked
S31603
316L SST
Liquid
0.001 (0.04)
Unattacked
S31603
316L SST
Liquid
0.004 (0.17)
Unattacked
N10276
Alloy C-276
Solid
0.003
Unattacked
N10276
Alloy C-276
Liquid
0.002 (0.07)
Unattacked
N10276
Alloy C-276
Liquid
0.002 (0.08)
Unattacked
N06625
Alloy 625
Solid
0.002 (0.07)
Unattacked
N06625
Alloy 625
Liquid
0.006 (0.24)
Unattacked
N06625
Alloy 625
Liquid
0.0013 (0.04)
Unattacked
N08367
Alloy 6XN
Solid
0.001 (0.04)
Unattacked
N08367
Alloy 6XN
Liquid
0.0013 (0.04)
Unattacked
N08367
Alloy 6XN
Liquid
0.0013 (0.04)
Unattacked
(0.12)
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Corrosion Tests in Chlorine Dioxide Bleach
Environments
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Discussion and Conclusions
•  Accelerated Corrosion of Alloy C276 in Comparison to Both Type 316L
stainless steel and Alloy 625 When Exposed to the Actual Process
Conditions
•  Studies Have Shown That Ni-Cr-Mo Alloys Corrode at Higher Rates
Than Alloys Containing Less Chromium and Moly in Oxidizing Bleach
Manufacturing Processes (1)
•  Vanadates Can be Highly Oxidizing, Similar to Chlorine in Bleach
Manufacturing Processes
•  It May Be Possible to Get Localized Low pH Conditions Under Vanadate
Containing Deposits
•  Possible to Get Highly Oxidizing Acidic Conditions Under Deposits
Packed Against the Alloy C276 Diaphragm
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1. Allfonsson, Tuveson-Carlström, Wallén, Corrosion in Chlorine Dioxide Bleach Environments – Experiences with Stainless Steel and Nickel-Based Alloys ,
TAPPI Engineering Conference, September 1993, Orlando FL
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Discussion and Conclusions
•  Current Diaphragms Are Made Of Alloy 625 And Have
Been In Service For 2 Years
•  Still Reviewing Data To Determine If There Is A Better
Option Than Alloy 625 Or Type 316L Stainless Steel
•  High Chrome, Low Nickel and Moly
•  Duplex Stainless Steels (Alloy 2304, 2003, 2205)
•  Precipitation Hardening Stainless Steel (17-7PH)
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Acknowledgements
Brian Saldanha – DuPont, Wilmington DE
Mike Rogillio – DuPont, LaPorte
Chris Michna – DuPont, Houston
Sheryll Fox-Loughin – DuPont, Wilmington
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QUESTIONS?
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