U.S. patent application number 15/224692 was filed with the patent office on 2018-02-22 for surface metallographic method for characterizing the degree of sensitization of aluminum-magnesium alloys.
This patent application is currently assigned to The Government of the United States of America, as represented by the Secretary of the Navy. The applicant listed for this patent is The Government of the United States of America, as represented by the Secretary of the Navy, The Government of the United States of America, as represented by the Secretary of the Navy. Invention is credited to Ronald L. Holtz, Derek Horton.
Application Number | 20180053316 15/224692 |
Document ID | / |
Family ID | 61189030 |
Filed Date | 2018-02-22 |
United States Patent
Application |
20180053316 |
Kind Code |
A1 |
Holtz; Ronald L. ; et
al. |
February 22, 2018 |
SURFACE METALLOGRAPHIC METHOD FOR CHARACTERIZING THE DEGREE OF
SENSITIZATION OF ALUMINUM-MAGNESIUM ALLOYS
Abstract
A non-destructive method for assessing the "degree of
sensitization" of ship structures formed from aluminum-magnesium
marine service alloys. Features of the method include (1) selective
etching of beta phase in a sensitized aluminum-magnesium alloy (2)
metallographic recording of the etched surface; (3) image
enhancement to produce high-contrast binary images of etched and
unetched areas; (4) image analysis of the enhanced images using
line segments along grain boundaries to provide statistical
information about the grain boundary beta phase percentage and (5)
calibration, whereby the grain boundary beta phase percentage is
converted to an expression of the degree of sensitization in the
sample.
Inventors: |
Holtz; Ronald L.; (Lorton,
VA) ; Horton; Derek; (Alexandria, VA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Government of the United States of America, as represented by
the Secretary of the Navy |
Arlington |
VA |
US |
|
|
Assignee: |
The Government of the United States
of America, as represented by the Secretary of the Navy
Arlington
VA
|
Family ID: |
61189030 |
Appl. No.: |
15/224692 |
Filed: |
August 1, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62202190 |
Aug 7, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06T 2207/30136
20130101; C23F 1/02 20130101; G06T 7/0004 20130101; G06T 7/13
20170101; G06T 7/41 20170101; G06T 7/12 20170101; G06T 7/11
20170101; G06T 2207/10056 20130101; G06T 7/0008 20130101; C23F 1/00
20130101; G06T 7/90 20170101; G06T 2207/20112 20130101; G06T
2207/10024 20130101 |
International
Class: |
G06T 7/40 20060101
G06T007/40; G06T 7/00 20060101 G06T007/00 |
Claims
1. A method for characterizing a degree of sensitization of an
aluminum-magnesium alloy sample, comprising: polishing a surface of
the sample; applying an etchant to the polished surface, the
etchant selectively etching beta phase Mg.sub.2Al.sub.3 present in
the grain boundaries of the sample; making a digital metallographic
image of the etched surface of the sample; enhancing the
metallographic image to produce a binary black-and-white digital
image of the etched surface, wherein the areas of the image
representing the etched beta phase Mg.sub.2Al.sub.3 appear in the
image as black spots formed from black pixels and the unetched
areas of the surface appear in the image as white areas formed from
white pixels; applying a series of linked line segments to the
enhanced image to connect a plurality of black spots in the image
into a single line, each of the line segments connecting two black
spots in the image and having a corresponding linewidth of from 1
pixel to a width of the smallest of the plurality of black spots to
be linked, wherein the single line delineates a grain boundary of
the sample having etched beta phase Mg.sub.2Al.sub.3; applying a
statistical distribution analysis to the black and white pixels
along the linked line segments to determine a ratio of black pixels
to white pixels along the linked line segment; calculating an
average percentage of black pixels along the linked line segment,
wherein an average percentage of black pixels along the linked line
segment is indicative of the degree of sensitization of the sample;
repeating the steps of linking the line segments to delineate all
of the grain boundaries present in the image, and for each of the
linked line segments, repeating the steps of applying the
statistical distribution analysis to each of the delineated grain
boundaries and calculating the average percentage of black pixels
along the lined line segment; totaling all of the calculated
average percentages of black pixels for all of the linked line
segments in the image and calculating an image-wise average of the
total percentage of black pixels versus white pixels on grain
boundaries in the image; and converting the image-wise average of
the percentage of black pixels versus white pixels on plurality of
grain boundaries to an expression of the degree of sensitization of
the sample.
2. The method according to claim 1, wherein the etchant is ammonium
persulfate.
3. The method according to claim 1, wherein the etchant is ammonium
persulfate at a 0.2 M concentration adjusted to a pH of 1.2, the
etching taking place in a temperature controlled bath at a
temperature of about 35.degree. C. for about 60 minutes.
4. The method according to claim 1, wherein the etching occurs over
a time period of about 20 to about 100 minutes.
5. The method according to claim 1, wherein the etching occurs over
a time period of about 60 minutes.
6. The method according to claim 1, wherein the digital
metallographic image is an optical micrograph of the sample, the
optical micrograph having a magnification of about 10.times. to
about 100.times..
7. The method according to claim 6, wherein the digital
metallographic is an optical micrograph having a magnification of
about 20.times. to about 50.times..
8. The method according to claim 1, wherein the digital
metallographic is an optical micrograph recorded in a TIFF format
having a resolution of about 150 dpi.
9. The method according to claim 1, wherein the width of all of the
line segments is the same.
10. The method according to claim 1, wherein the width of the line
segments varies, a width of any single line segment corresponding
to a width of the two black spots which the line segment connects.
Description
CROSS-REFERENCE
[0001] This Application is a Nonprovisional of and claims the
benefit of priority under 35 U.S.C. .sctn.119 based on U.S.
Provisional Patent Application No. 62/202,190 filed on Aug. 7,
2015. The Provisional Application and all references cited herein
are hereby incorporated by reference into the present disclosure in
their entirety.
TECHNICAL FIELD
[0002] The present invention relates to aluminum-magnesium alloys,
specifically to a method for characterizing the degree of
sensitization of such alloys.
BACKGROUND
[0003] Heat sensitization of aluminum 5xxx aluminum-magnesium
alloys is associated with the formation of a magnesium-rich
(compared to the solid solution) Mg.sub.2Al.sub.3 phase on the
grain boundaries when the alloy is exposed to elevated
temperatures. This magnesium-rich phase, known as beta phase
Mg.sub.2Al.sub.3, or often simply as "beta phase," on the grain
boundaries is anodic with respect to the surrounding
aluminum-magnesium solid solution, thus presence of beta phase on
the grain boundaries increases the potential for intergranular
corrosion, stress-corrosion cracking, and intergranular
corrosion-fatigue, leading to degradation of ship structure
mechanical reliability.
[0004] Conventionally, the degree of sensitization (DOS) is
characterized with the ASTM G67 Nitric Acid Mass Loss Test set
forth in ASTM G67-13, "Standard Test Method for Determining the
Susceptibility to Intergranular Corrosion of 5XXX Series Aluminum
Alloys by Mass Loss After Exposure to Nitric Acid (NAMLT Test),"
(2013) available from ASTM International, Inc. In this test, a
specimen of the material in question is immersed in
temperature-controlled concentrated nitric acid for a period of
time, and the amount of mass lost from the specimen after the test
versus before the test is measured. Essentially, the test is
contrived to allow the acid to severely etch the grain boundaries,
and the result is that grains fall out, accounting for the mass
loss. Obviously this approach is destructive and generates
undesirable hazardous waste, and so cannot be used in-situ on a
ship.
[0005] The total amount of beta phase present in the worst case of
sensitization is very small, while the effects on corrosion of
grain boundaries is very high. A number of approaches for assessing
the DOS based on measures of the amount of beta phase in the bulk
material have been tried or speculated about, including use of
microwave cavity resonance perturbation, electrical conductivity,
hardness, ultrasonic attenuation, x-ray composition analysis, and
so on. See, e.g., http://www.alphasense.net/nde.html (microwave
cavity resonance perturbation); C. Chukunonye, "Sensitization
Characterization of 5083 and 5456 Aluminum Alloys using
Ultrasound," Dissertation, University of Louisiana at Lafayette
(2015) ("ultrasonic method"); M. Shedd, G. Bunget, F. Friedersdorf,
and N. Brown, "Embedded Long Service Life Monitoring System for
Aluminum Alloy Sensitization," ASNE MegaRust Conference, (2013)
("eddy current method"); and B. A. Shaw, "Fieldable Probe for
Quantitative Assessment of Degree of Sensitization in Marine
Aluminum Alloys," 2009 Navy SBIR Topic N09-T022, Award 90313 (2009)
("x-ray diffraction method).
[0006] Certain mechanical properties of the alloys, such as
hardness, also correlate with the DOS. See I. N. A. Oguocha, O. J.
Adigun, and S. Yannacopoulos, "Effect of sensitization heat
treatment on properties of Al--Mg alloy AA5083-H116," J. Matter.
Sci. (2008) 43:4208-4214. These correlations also could form the
basis of a DOS measurement approach. Hardness in particular is a
very easy measurement to make. Researchers at the Naval Research
Laboratory have done exploratory research on using hardness
measurements for estimating DOS. The hardness, for example Rockwell
Hardness or Vicker's Hardness, decreases rapidly with degree of
sensitization. However this approach requires a known unsensitized
reference sample of the material being tested, which is not always
available.
[0007] In addition, beta phase can exist intragranularly in
addition to being on the grain boundaries, though the intragranular
beta phase does not affect the intergranular corrosion
significantly. Thus techniques that seek to assess DOS based on
measures of the amount of beta phase in the bulk tend to be either
insufficiently sensitive, or yield overestimates of the effect of
grain boundary beta.
[0008] An alternative to the ASTM G67 DOS test is the portable
electrochemical system known as the ElectraWatch DoS Probe
developed under funding from the Department of the Navy. See
Electrawatch, Inc., Degree of Sensitization (DoS) Probe, which can
be found at http://www.electrawatch.com/DoS.html; see also W. J.
Golumbfskie, K. T. Tran, J. M. Noland, R. Park, D. J. Stiles, G.
Grogan, and C. Wong," Survey of Detection, Mitigation, and Repair
Technologies to Address Problems Caused by Sensitization of Al--Mg
Alloys on Navy Ships," CORROSION, Vol. 72, No. 2, pp. 314-328. The
ElectraWatch DoS Probe works by measuring the electrochemical
currents and voltages as the beta phase undergoes reaction to a
reagent is applied to the surface. While this works well in the
laboratory, it is a specialized instrument that must have access to
a large flat area of surface to seal against, to contain the
electrochemical reaction volume. Further, it can be temperature
sensitive and requires specially trained personnel to operate.
[0009] A viable DOS characterization tool must be specific to grain
boundary beta phase. Microstructure analysis studies by NRL, see R.
Goswami and R. L Holtz, "Transmission Electron Microscopy
Investigations of Grain Boundary Beta Phase Precipitation in Al
5083 Aged at 373K," Metallurgical and Materials Transactions A,
44A, pp 1279-1289 (2013), and others have shown that high values of
DOS as measured by the mass loss test are directly associated with
the degree of coverage of the grain boundaries by beta phase. When
the beta phase is present primarily as isolated, precipitates, the
G67 mass loss is very low. When the beta phase is a continuous or
nearly continuous layer, the mass loss is very high. While the
exact role the continuity or topology of the beta coverage plays in
G67 mass loss is not entirely known, it is sufficiently clear that
measuring beta coverage of the grain boundaries provides a
metallographic option for estimating DOS, if calibrated against
G67. Such an approach would be non-destructive to the bulk of the
material, thus could be used in-situ in some circumstances, and
eliminates the generation of large amounts of acid waste.
[0010] Metallographic etching can be used to dissolve beta phase in
aluminum-magnesium alloys, and if the beta phase is concentrated
along grain boundaries, the etching patterns are easily seen with
optical metallography. Qualitative assessment of sensitization via
etching and metallography has been commonly done since the
sensitization phenomenon was first identified. This is suggested in
ASTM B928; see also S. Jain, J. L. Hudson, and J. R. Scully,
"Effects of constituent particles and sensitization on surface
spreading of intergranular corrosion on a sensitized AA5083 alloy,"
Electrochimica Acta 108 (2013) 253-264. However, prior to the
current invention disclosure, no quantitative metallographic
technique has been developed for this purpose.
SUMMARY
[0011] This summary is intended to introduce, in simplified form, a
selection of concepts that are further described in the Detailed
Description. This summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to be used as an aid in determining the scope of the
claimed subject matter. Instead, it is merely presented as a brief
overview of the subject matter described and claimed herein.
[0012] The present invention provides a method for assessing the
"degree of sensitization," or "DOS," of ship structures formed from
aluminum-magnesium marine service alloys. The method of the present
invention involves the use of a very light acid etch which only
slightly affects the surface and does not damage the structural
integrity of the metal, and so may be described as being
"non-destructive."
[0013] The method of the present invention includes the following
steps/features:
[0014] (1) Selective etching of the beta phase in a sensitized
aluminum-magnesium alloy to prepare the surface for microscopic
examination and to dissolve small amounts of the beta phase, which
creates visible contrast between the beta phase and the rest of the
alloy;
[0015] (2) Metallographic recording of the etched surface, e.g., by
means of optical microscopy at magnifications ranging from
10.times. to 100.times.;
[0016] (3) Image enhancement to produce high-contrast binary images
of etched and unetched areas in the metallographic image of the
etched sample, where the etched, i.e., dissolved, beta phase areas
appear in the image as spots formed from black pixels and the
unetched portions of the sample are shown by white pixels; and
[0017] (4) Image analysis of the enhanced images using line
segments connecting black spots in the image to delineate a grain
boundary to provide statistical information about the etched versus
the unetched areas, where the average grain boundary black spot
density along a grain boundary delineated by the line segments
represents an estimate of the percentage of that grain boundary
covered by beta phase.
[0018] Step (4) is then repeated a number of times to delineate a
plurality of grain boundaries shown in the image. Once a number of
grain boundaries are delineated, the next step is
[0019] (5) Calibration, whereby the grain boundary beta phase
percentage is converted to an expression of the degree of
sensitization in the sample. Grain boundary black spot densities of
less than 60% do not reflect any significant degree of
sensitization in the material, while for densities greater than 60%
there is a nearly linear correlation between the density and the
DOS such that the DOS can be easily determined once the grain
boundary black spot density is found.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIGS. 1A and 1B are optical metallographs depicting an
aluminum-magnesium alloy sample that has been etched in accordance
with the method of the present invention, where FIG. 1A is the
original micrograph image and FIG. 1B is the image after it has
been thresholded and converted to a black-and-white-binary
image.
[0021] FIG. 2 is an enlargement of the boxed portion of the image
in FIG. 1B in which line segments are applied to delineate a grain
boundary in accordance with the method of the present
invention.
[0022] FIG. 3 is an exemplary thresholded binary optical
metallograph having an average grain boundary beta phase density of
about 75% as estimated using the method in accordance with the
present invention.
[0023] FIG. 4 is a plot showing the results of a preliminary
calibration of the degree of sensitization versus average grain
boundary beta phase density estimated using the method in
accordance with the present invention.
DETAILED DESCRIPTION
[0024] The aspects and features of the present invention summarized
above can be embodied in various forms. The following description
shows, by way of illustration, combinations and configurations in
which the aspects and features can be put into practice. It is
understood that the described aspects, features, and/or embodiments
are merely examples, and that one skilled in the art may utilize
other aspects, features, and/or embodiments or make structural and
functional modifications without departing from the scope of the
present disclosure.
[0025] The present invention provides a method for assessing the
"degree of sensitization," or "DOS," of ship structures formed from
aluminum-magnesium marine service alloys conforming to the
standards set by ASTM International standard ASTM B928. See
"Standard Specification for High Magnesium Aluminum Alloy Sheet and
Plate for Marine Service and Similar Environments," ASTM B928-15,
ASTM International, Inc., (2015). The method of the present
invention involves the use of a very light acid etch which only
slightly affects the surface and does not damage the structural
integrity of the metal, and so may be described as being
"non-destructive."
[0026] The method of the present invention includes the following
steps/features:
[0027] (1) Selective Etching of Beta Phase in Sensitized
Aluminum-Magnesium Alloy.
[0028] In a first step, the surface of a sensitized
aluminum-magnesium material sample to be analyzed is polished and
then is etched. This etching step prepares the surface for
microscopic examination and dissolves small amounts of the beta
phase in the material to create a visible contrast between the beta
phase and the rest of the material. In most cases, the samples will
be etched using ammonium persulfate, which has been shown to be a
highly selective etch for beta phase in aluminum-magnesium alloy
materials, see J. Buczynski, "Electrochemical analysis if etchants
used to detect sensitization in marine grade 5xxx
aluminum-magnesium alloys," M. S. Thesis, University of Virginia
(2012), in a temperature-controlled bath. Etching times can range
from 20 to 100 minutes, with the best etching time of approximately
60 minutes corresponding to the time after which no significant
changes occur. In an exemplary embodiment analyzed for this
disclosure, this etching step was performed using ammonium
persulfate at a 0.2 M concentration adjusted to a pH of 1.2 in a
temperature-controlled bath at a temperature of 35.degree. C. for
approximately 60 minutes, though other suitable etchants and
etching conditions may be used as appropriate.
[0029] (2) Metallographic Recording of the Polished and Etched
Surface.
[0030] In a next step, a metallographic recording of the polished
and etched surface is made, for example, by means of optical
microscopy at magnifications ranging from 10.times. to 100.times..
The best results are obtained when a large number of grain
boundaries are contained in an image, to ensure good statistical
confidence, but at sufficient magnification to resolve particles
and separations between particles down to less than 1 micrometer in
size. For the aluminum alloy samples used for our demonstration,
the best results have been obtained with magnifications of
20.times. to 50.times., recording the images in high-resolution
(150 dpi) TIFF format, though any suitable graphics format or
resolution can be used as appropriate. A metallograph of an
exemplary etched sample is shown in FIG. 1A.
[0031] (3) Image Enhancement to Produce High-Contrast Binary Images
of Etched and Unetched Areas.
[0032] The next step involves enhancement of the optical images of
the etched sample to produce high-quality binary, i.e.,
black-and-white, images of the etched sample, where the where the
etched, i.e., dissolved, beta phase areas appear in the image as
spots formed from black pixels and the unetched portions of the
sample are shown by white pixels. Enhancement of the image can be
achieved using any suitable image manipulation software such as the
Fiji or ImageJ applications known in the art. See J. Schindelin, et
al., "Fiji: an open-source platform for biological-image analysis,"
Nature methods, 9(7), 676-682 (2012); and C. A. Schneider, W. S.
Rasband, and K. W. Eliceiri, "NIH Image to ImageJ: 25 years of
image analysis," Nature methods, 9(7), 671-675 (2012).
[0033] Thus, in this step, the raw images are first converted to a
greyscale format, e.g., an 8-bit greyscale format, and a
thresholding procedure is then applied to enhance the contrast
between etched areas and unetched areas in the image, particularly
along the grain boundaries. Any suitable thresholding procedure can
be used to enhance the contrast, such as the ImageJ.net Auto Local
Threshold procedure. See G Landini, et al., Auto Local Threshold,
http://fiji.sc/wiki/index.php/Auto_Local_Threshold#Bernsen ("Auto
Local Threshold"). Irrespective of the method used, the ideal
thresholding procedures are those for which the final beta phase
coverage results are not sensitive to thresholding parameters. The
most consistent results obtained by the inventors used an automatic
local threshold process with a radius of 5 to 15 pixels, using the
Bernsen or Otsu methods described in Auto Local Threshold,
supra.
[0034] After the thresholding operation, the image is converted to
pure binary format with a white background, which results in black
and white images where the etched, i.e., dissolved, beta phase
areas appear in the image as spots formed from black pixels and the
unetched portions of the sample are shown by white pixels. FIG. 1B
shows the result of this thresholding and binary conversion of the
image shown in FIG. 1A, which has converted the gray-scale tones of
FIG. 1A into a clear black and white image.
[0035] (4) Image Analysis of the Enhanced Images Using Line
Segments Along Grain Boundaries.
[0036] In the next step, the enhanced images are analyzed to
provide statistical information about the etched versus the
unetched areas. In this analysis, the average "black" value along a
grain boundary represents an estimate of the percentage of grain
boundary covered by beta phase.
[0037] To perform this image analysis, a series of linked straight
or curved line segments are used to connect the black spots in the
image corresponding to the etched, i.e., dissolved, grain boundary
beta phase particles. The endpoint of each line segment is located
in the middle of a black spot in the image, with a length and a
curvature of the line segments being configured so that they
sequentially connect a plurality of black spots in the image to
form a single line that represents a grain boundary. The line
segments are configured so that the line goes through the middle of
the black spots to be linked, and can have any appropriate
linewidth, from 1 pixel up to the width of the smallest linked
black spot. In some embodiments, the linewidths can all be the same
for a given linked line, while in other embodiments, the linewidths
can vary depending on the size of the spots to be connected, though
care should be given since linewidths that are too large capture
too much white space in the image while linewidths too narrow risk
missing smaller dark spots. In many cases, the best linewidths can
be found by trying increasing values starting from 1 pixel and
determining the range of linewidths from which a resulting beta
phase coverage estimate is reproducible.
[0038] FIG. 2 provides an illustrative example, and depicts an
enlarged view of the boxed portion of the image in FIG. 1B in which
one grain boundary is shown by a series of line segments connecting
the chain of black spots delineating the grain boundary. In the
exemplary case shown in FIG. 2, a series of linked line segments
having a linewidth of 3 pixels were used, where the endpoint of
each line segment is located in the middle of a corresponding black
spot representing an etched, i.e., dissolved, grain boundary beta
phase particle.
[0039] This process of using connected line segments to delineate a
grain boundary is then repeated a number of times to delineate all
of the grain boundaries in the image.
[0040] Next, for each delineated grain boundary, the statistical
distribution of black versus white pixels along the grain boundary
is calculated, using any suitable graphics processing methodology
such as the ImageJ application described above, and the average
black pixel coverage for that grain boundary is determined, where
the average black pixel coverage for a grain boundary is associated
with the average beta phase present in that grain boundary. While
the correspondence is not exact (the etched areas tend to be
slightly bigger than the actual beta phase particles) we find a
clear systematic relationship between average coverage of the black
vs. the white pixels and the bulk ASTM G67 DOS values that serve as
a calibration as discussed below. Thus, based on this statistical
analysis, it was determined that the grain boundary delineated by
the line segments shown in FIG. 2 has a black density of 48%, with
the average black density for all of the grain boundaries in the
image being 50%, while the sample shown in the image in FIG. 3 has
a higher concentration of grain boundary beta phase particles,
about 75%.
[0041] In some embodiments, the statistical distribution of black
versus white pixels and average black pixel coverage can be
calculated for a first grain boundary before a second grain
boundary is delineated, while in other embodiments, the statistical
distribution and average black pixel coverage can be calculated
after all of the grain boundaries in the image are delineated. In
addition, while in some embodiments, the line segments are
connected to delineate multiple, separate grain boundaries, it may
be possible to use the line segments connect all of the grain
boundaries in the image in a single line, and in such a case, only
one calculation for each of the statistical distribution and the
black pixel coverage is needed, with the calculation of black pixel
coverage serving as the image-wise average of total black pixel
grain boundary coverage referenced below.
[0042] (5) Calibration.
[0043] The average black pixel coverage for all grain boundaries in
the image is then totaled and this total is averaged over the
number of delineated grain boundaries in the image to produce an
image-wise average of the total black pixel grain boundary coverage
in the image, where the image-wise average is associated with the
grain boundary beta phase percentage present in all of the grain
boundaries in the image. This image-wise average is then converted
to an expression of the degree of sensitization in the sample.
Measured correlation between the grain boundary beta phase
percentage and the degree of sensitization measured by the standard
ASTM G67 method gives the calibration for converting measured beta
phase coverage to DOS values. Calibration was performed by
comparing the beta phase coverage determined by the image analysis
method described above on samples that had known ASTM G67 values
from other measurements.
[0044] An initial calibration is shown in the plot in FIG. 4. The
known ASTM G67 DOS for a sample having an average grain boundary
beta phase particle density of 65% is 0, while a sample having a
grain boundary beta phase particle density of 75% estimated in
accordance with the present invention had a measured DOS of about
10 mg/cm.sup.2 and a sample having a grain boundary beta phase
particle density of 90% estimated in accordance with the present
invention had a measured DOS of about 40 mg/cm.sup.2. Thus, as can
be seen from the plot in FIG. 4, grain boundary beta phase particle
densities of less than 60% do not reflect any significant degree of
sensitization in the material, while for densities greater than 60%
there is a nearly linear correlation between the density and the
DOS such that the DOS can be easily determined once the grain
boundary beta phase particle density is found.
[0045] We note that a limitation of the estimation method of the
present invention occurs when the amount of grain boundary beta
phase present is very low. When it is very low, it is difficult to
identify with certainty what features are grain boundaries, versus
other apparent but possibly random patterns of etched areas. In any
case, for practical purposes, very low beta concentrations are of
little interest, as the materials in such condition are effectively
unsensitized anyway.
[0046] Advantages and New Features
[0047] The method of the present invention can be used to analyze
the DOS of a metal sample by analyzing only the surface, and so
does not require cutting/removal of material from the subject
structure, and so is structurally non-destructive, in contrast to
the standard ASTM G67 method of the prior art.
[0048] In addition, the method of the present invention can be
applied to a single surface having any surface contour in any
orientation so long as the material surface can be polished, and so
can be used for both horizontal and vertical surfaces without
having to treat the entire structure.
[0049] Moreover, while the standard ASTM G67 test uses 100 CC of
concentrated nitric acid for every measurement, the method of the
present invention uses a fairly weak acid that just wets the
surface and so does not generate large quantities of hazardous
waste. The etching performed in the method of the present invention
does not penetrate significantly into the material and therefore
does not degrade the structural properties of the material or
remove mass, and the etched layer can be easily restored if needed
by polishing or sanding.
[0050] Finally, the method of the present invention uses commonly
available laboratory metallographic facilities and so is
inexpensive and can be readily performed without the need for
specialized equipment or facilities.
[0051] Alternatives
[0052] While the DOS estimation method in accordance with the
present invention can employ manual, i.e., visual, selection of the
grain boundaries in the enhanced images, it may also be possible to
use computational algorithms can be used to automatically find and
select the grain boundaries.
[0053] Any suitable image thresholding procedure and line segment
criteria can be used, provided these other approaches do not
artificially over- or under-estimate the grain boundary beta phase
coverage.
[0054] Any suitable etchant, etching temperature, and/or etching
time can be used. For example, while not as highly specific as the
ammonium persulfate described herein, any known beta phase etchant
can be used for aluminum-magnesium alloys.
[0055] In addition, the DOS estimation method of the present
invention can be used for other metallic alloys that also exhibit
sensitization phenomena, for example corrosion resistant steels,
provided that suitable etchants are available.
[0056] Thus, the present invention provides a simple, inexpensive,
and easily scalable method for estimating the degree of
sensitization of metal alloys such as aluminum-magnesium alloys
used in marine environments, and provides an alternative to the
conventional methods set forth in ASTM G67-13.
[0057] Although particular embodiments, aspects, and features have
been described and illustrated, it should be noted that the
invention described herein is not limited to only those
embodiments, aspects, and features, and it should be readily
appreciated that modifications may be made by persons skilled in
the art. The present application contemplates any and all
combinations and modifications to those embodiments described and
claimed herein, and all such combinations and embodiments are
within the scope and spirit of the present disclosure.
* * * * *
References