Instruction Manual for the Degree of Sensitization (DoS)
Probe
ElectraWatch, Inc. SUITE 102, 660 Hunters Place Charlottesville, VA 22911 USA
518 312-2443 434 970-7878
[email protected] www.electrawatch.com
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Table of Contents Table of Contents............................................................................................................................ 2
Background..................................................................................................................................... 3
5XXX Aluminum Sensitization.................................................................................................. 3
Theory of Operation:................................................................................................................... 5
Use of the DoS Probe:..................................................................................................................... 7
Use of the DoS Probe:..................................................................................................................... 8
Surface Preparation..................................................................................................................... 8
Mount DoS probe over area of interest....................................................................................... 8
Prepare DoS probe for measurement .......................................................................................... 8
After the measurement is completed: ....................................................................................... 10
Data Analysis ................................................................................................................................ 12
References:.................................................................................................................................... 12
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Background
5XXX Aluminum Sensitization
Figure 1. Al-Mg phase diagram. The shaded area corresponds to the composition of the 5XXX Naval alloys.
Figure 2. Cross-sectional and plane view micrographs of unsensitized (left) and very sensitized (right) material after
exposure.
To maximize the stability and, in some cases, speed of some Naval and commercial vessels, aluminum-magnesium alloys are used to construct ship superstructures and other structures. The primary metal, aluminum, offers the advantage of being lightweight; adding magnesium to the aluminum creates an alloy that is both lightweight and strong. In the tempers commonly used, H116 and H321, the aluminum alloy matrix is supersaturated with magnesium to maximize strength. However, this condition is thermodynamically metastable, as indicated in the Al-Mg phase diagram (Figure 1). Exposure to elevated temperatures (above 60° C) for extended periods of time causes the magnesium, which should be evenly dispersed within the aluminum matrix, to form beta-phase (Mg2Al3) precipitates. These precipitates are strongly anodic to the Al-Mg solid solution1 and will cause galvanic corrosion2 in marine atmospheric conditions (Figure 2).2 The formation of this precipitate is called sensitization. If the precipitates form along the grain boundaries of the material as a connected network intergranular corrosion, exfoliation, and stress corrosion cracking (SCC,
) can occur. Consequently, sensitization is a serious issue for the Navy and other operators of 5XXX
Figure 3
3
4
Figure 3. Example of stress corrosion cracking on USS CAPE ST. GEORGE (CG-71) caused by aluminum sensitization.
aluminum ships.
The conventional laboratory approach to determine the degree of sensitization (DoS) is ASTM G67 (Figure 4) which involves excising a coupon from the ship’s structure, immersion of this coupon in nitric acid for 24 hours and measuring the mass loss resulting from intergranular corrosion. If the mass loss is greater than 25 mg/cm2, the material is sensitized and fails the G67 test. If the mass loss is less than 15 mg/cm2, the material is not sensitized and passes the test.
The obvious problems with the G67 test for ship inspection are that it is destructive, requires laboratory testing of coupons, is expensive, and is labor and time intensive.
The ElectraWatch, Inc., Degree of Sensitization (DoS) Probe is a nondestructive instrument designed to determine DoS in the field to assist in maintenance decisions and guide predictions of structural integrity or health in service.3 Accordingly, it can be used to
• Reduce maintenance costs of the service lifetime of the ship
ASTM G67 • 2” x 1/4” x 1/4” sample
• Immersed 24 hours in 70% nitric acid at 30°C
• Intergranular corrosion (IGC)
• Mass loss characterizes Degree of Sensitization (DoS)
>25 mg/cm2 = fail
15 – 25 mg/cm2 = indeterminate
< 15 mg/cm2 = pass
• Higher DoS linked to higher intergranular stress corrosion crack (IGSCC) velocity
Figure 4. ASTM G67 Test.
• Minimize repair times
• Increase fleet readiness.
The ElectraWatch DoS probe is a battery-powered, hand-held device with an internal micro-fluidics subsystem and printed circuit board (PCB) that implements the electrochemical procedure identified in the Theory of Operation. The probe transmits the results of the DoS measurement to a laptop computer or similar device where they can be analyzed and displayed to the user via a Graphical User Interface (GUI).
Theory of Operation: Aluminum and magnesium exhibit different electrochemical properties. Figure 6 shows a clear example of these differences where the beta phase exhibits much higher currents (by several orders of magnitude) at a given potential than does the quenched alloy with no precipitates.. As can be seen in the superimposed Pourbaix Diagrams of aluminum and magnesium (Figure 5), under alkaline conditions, aluminum will dissolve while magnesium will passivate. The buffer solution in the probe is an alkaline (pH 10.8) solution. The buffer solution minimizes corrosion of the aluminum alloy and allow electrochemical measurements to distinguish between the unsensitized aluminum alloy and the sensitized alloy with beta-phase precipitates
The DoS probe utilizes this difference in electrochemistry ( ) to detect beta phase precipitates. Specifically, it detects the differences in electrochemcial behavior in a 0.2M phosphate buffer at pH 10.8. Figure 8 schematically shows the current DoS probe electrochemical procedure.
Figure 7
Figure 9 shows the decay of the open circuit potential (OCP, aka Ecorr) as a function of time in a pH 10.8 0.2M phosphate buffer for 5456 aluminum alloy for several different DoS values.
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Figure 5. Superimposed Pourbaix Diagrams for Al and Mg. The red line represents a possible buffer pH level.
E (V
vs.
SC
E)E
(V v
s. S
CE)
Figure 6. Polarization curves of the solution-heat-treated and quenched 5083Al alloy and the beta (Mg3Al2) phase.
The OCP of sensitized material decreases to its final value much earlier than that of the non-sensitized material.
Figure 10 shows the time for the OCP to decrease to -1.2V (MSE) for both 5083 and 5456 aluminum alloys as functions of DoS values. In these graphs, the ordinate (y) scales for the two alloys have been adjusted to show that the alloys exhibit identical trends. Both the precondition current and OCP decay exhibit near linear trends with DoS. Observing both the
precondition current and the OCP decay allows improved determination of sensitization than observing either parameter alone.
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Monitor OCP decay for 15 min, determine time‐to‐ ‐1.2V(MSE)
Polarize to ‐0.1 V(MSE) for 30 min, monitor current density
Measure polarization resistance
Immerse, monitor OCP for 30 min
Monitor OCP decay for 15 min, determine time‐to‐ ‐1.2V(MSE)
Polarize to ‐0.1 V(MSE) for 30 min, monitor current density
Measure polarization resistance
Immerse, monitor OCP for 30 min
Figure 8. Schematic of alkaline electrochemical procedure.
Current Density (A/cm2)
Pot
entia
l (V
vs
HgS
O4)
Pure Mg3Al2
Unsensitized 5456 DoS=2.6 mg/cm2
Sensitized 5456 DoS=75 mg/cm2
10-310-410-510-610-710-8
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
-1.7
Current Density (A/cm2)
Pot
entia
l (V
vs
HgS
O4)
Pure Mg3Al2
Unsensitized 5456 DoS=2.6 mg/cm2
Sensitized 5456 DoS=75 mg/cm2
10-310-410-510-610-710-8
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
-1.7
Figure 7. Potentiodynamic polarization of unsensitized 5456 Al (DoS=2.6 mg/cm2), sensitized 5456 Al (DoS=75 mg/cm2), and pure beta Mg3Al2.
AA5456‐H16 in pH 10.8 0.25 M phosphate bufferOCP Decay After 30 min at ‐0.1 V(MSE)
‐1.8
‐1.6
‐1.4
‐1.2
‐1
‐0.8
‐0.6
‐0.4
0 50 100 150 200 250 300 350 400 450 500
Time (s)
Potential (V) vs. HgSO
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SHTQ ‐‐ 2.6 mg/cm2
3 days/100 C ‐‐ 28.1 mg/cm2
7 days/100 C ‐‐ 47.6 mg/cm2
30 days /100 C‐‐ 74.8 mg/cm2
Figure 9. OCP as a function of time in pH 10.8 0.2M phosphate buffer for two 5XXX alloys with different DoS.
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Precondition Current
3.0E-05
3.5E-05
4.0E-05
4.5E-05
5.0E-05
5.5E-05
6.0E-05
0 10 20 30 40 50 60 70 80
DoS (mg/cm2)
5456
Cur
rent
Den
sity
(A/c
m2 )
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
2.0E-04
2.2E-04
5083
Cur
rent
Den
sity
(A/c
m2 )
54565083
OCP Decay
100
120
140
160
180
200
220
240
0 10 20 30 40 50 60 70 80
DoS (mg/cm2)
5456
Tim
e fo
r OC
P =
-1.2
V,H
gSO
4 (s)
160
170
180
190
200
210
220
230
240
5083
Tim
e fo
r OC
P =
-1.2
V,H
gSO
4 (s)
54565083
Figure 10. Precondition current and OCP decay for 5083 and 5456 as a function of DoS. The more complete (additional DoS values) 5456 data are from the parallel program. Note the difference in scale to show both alloys exhibit the same functional dependence.
Use of the DoS Probe:
Surface Preparation • Remove paint from the area (approximately 4” x 4”) being inspected.
• Sand with 800 grit and 1200 grit sand paper
Mount DoS probe over area of interest • If deck area is being measured
o Active suction by the suction cup units is optional if probe is stable
• If bulkhead area is being measured
o Orient the probe so that the air-water separator filter ( ) is at the top of the probe (12 o’clock position). In this position, the suction cups will form a normal “Y”.
Figure 11
Figure 11
o Apply compressor hose to power suction cups
o Activate suction cups
• Verify that the springs are in contact with bare metal (Figure 12).
• Adjust height of suction cups so that o-ring seal is tight against metal and level with metal
Prepare DoS probe for measurement • Load fluid container. Press down until
fully seated. Note the container is keyed so that the needle nozzles penetrate the seals on the fluid container (Figure 14). Tighten set screw.
• Load reference electrode. Attach the electrical connection with the allen screw (Figure 13).
Figure 11. Closeup of probe with the air-water separator filter circled
Figure 12. Closeup of probe showing springs indicated by arrows.
• Replace the air separator ( ).
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• Push lower button on probe twice (Figure 15).
• Enter location of measurement using lower button (cycles through space, 0, 1, 2, …) and upper button (advances to next digit). The file name gives both a date stamp and the entered location.
• Press lower button
• Press again if the file name (location) is correct. The probe will automatically check the seal. If the seal is satisfactory, the analysis chamber will begins to fill. If the seal is unsatisfactory, the display will
Figure 14. Fluid container showing orientation key, seals and needle nozzles on the probe for transfer of fluid from the container to the analytical chamber in the probe.
Figure 13. Reference electrode before and after installation into probe.
Key
Needle Nozzles
Seals
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indicate so. In this case, readjust the probe and try again. Possible causes of an unsatisfactory seal include: loose reference electrode, loose air-liquid separator, misalignment of probe nose over surface, or uneven surface.
• Green light blinks while measurements are being made (~75 minutes)
• Once measurement is completed (green light goes off) and electrolyte will automatically be transferred to fluid container.
After the measurement is completed:
• Remove fluid container. Mark as used.
• Remove air-liquid separator and dry with blast of compressed air. Replace with another air-liquid separator.
• Release suction cups.
• Wipe up any spilt electrolyte.
• If additional memory is available in the probe (DoS probe holds eight (8) measurements) and measurements at different locations are desired.
o Go to next location and repeat process. Note: Reference electrode can remain in probe.
• If DoS probe memory is full or measurements are complete.
o Connect DoS probe to computer and to battery recharging power supply (Figure 16).
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Figure 15. Electronics module of probe,
Computer USB
Power Supply Connection
Figure 16. Detail of top of electronics module showing connections for computer and power supply.
Lower Button
Upper Button
Display
o Open DoS probe GUI on computer (Figure 17). Click on RUN.
Figure 17. Screen shot of Probe GUI Collect and Erase Data page.
o Click on COLLECT DATA tab. Red light on DoS probe will turn on while data are being transferred.
o When data transfer is complete, red light on DoS probe will go out and file names will appear on GUI.
o To delete the data stored on the DoS probe (if desired), press ERASE DATA.
o Disconnect probe from computer. It is recommended to leave the DoS probe connected to the power supply to assure full charge for next measurement unless no additional measurements are anticipated for the near future.
o The data can be viewed by clicking on the GRAPH/ANALZE DATA tab (Figure 18). Each measurement is contained in a folder with three files. The first file is the initial OCP measurements; the second file is the second OCP measurements; the third file is the current during the potential hold period. The second file is the most useful in determining the DoS.
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Figure 18. Screen shot of Probe GUI Graph/Analyze Date page.
Data Analysis
References:
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1 D.A. Davis, and R.A. Hays, “Analysis of Failed Aluminum RAST Track Support Blocks and Posts From A CG-47 Class Guided Missile Cruiser” Tri-Services Corrosion Conference, 2006. 2 C. Wong, NAVSEA, 2008 3 P.E. Hess, “Structural Health Monitoring for High-Speed Naval Ships,” 6th Intl. Workshop on Structural Health Monitoring, 2007.
