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Frederick Morris Reinhart.

Corrosion of metals and alloys in the deep ocean online

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behavior of alloy 3003 appears to be depth (pressure)
dependent in that it increased in severity with
increasing depth.

6.3.3. Effect of Concentration of Oxygen

The corrosion rates, maximum pit depths, and
maximum depths of crevice corrosion on alloys 3003
and Alclad 3003 due to changes in the concentration
of oxygen in seawater were erratic.

6.3.4. Stress Corrosion

Alloy 3003-H14 was not susceptible to stress cor-
rosion when stressed at values equivalent to 50 and
75% of its yield strength and exposed at the
2,500-foot depth for 402 days as given in Table 67.

6.3.5. Corrosion Products

Corrosion products from alloy 3003-H14 were
analyzed by X-ray diffraction, spectrographic
analysis, quantitative chemical analysis, and infra-red
spectrophotometry. The qualitative results were:
amorphous A1 2 3 -XH 2 0, NaCl, Si0 2 , Al, Na, Si, Mg,
Fe, Cu, Ca, Mn, 3.58% chloride ion, 18.77% sulfate
ion, and considerable phosphate ion.

6.3.6. Mechanical Properties

The effects of exposure on the mechanical pro-
perties of alloys 3003-H14, Alclad 3003-H12, and
Alclad 3003-H14 are given in Table 68. In general,
the mechanical properties of alloys 3003-H14 and
Alclad 3003-H12 were adversely affected by exposure
at depth.



6.4. 5000 SERIES ALUMINUM ALLOYS
(ALUMINUM-MAGNESIUM ALLOYS)

The chemical compositions of the 5000 Series
aluminum alloys are given in Table 69, their corrosion
rates and types of corrosion in Table 70, their stress
corrosion behavior in Table 71, and the effect of
exposure on their mechanical properties in Table 72.

Aluminum is alloyed with magnesium to form an
important class of nonheat-treatable alloys (5000
Series). Their utility and importance are based on
their resistance to corrosion, high strength without
heat treatment, and good weldability.

The 5 000 Series aluminum alloys corroded
chiefly by the crevice and pitting types of localized
corrosion. Other types of corrosion found were:
blistering, crater, edge, intergranular, line, and
exfoliation.

6.4.1. Duration of Exposure

The general effect of duration of exposure on the
corrosion of the 5000 Series alloys was erratic and
nonuniform. The corrosion rates and the maximum
depths of pitting or crevice corrosion neither
increased nor decreased consistently with increasing
duration of exposure; in many cases, the behavior was
erratic.

6.4.2. Effect of Depth

After 1 year of exposure the average corrosion
rates of all the 5000 Series alloys increased with
depth, but not linearly. Also, the maximum depths of
pits of all the alloys increased linearly with depth.
The maximum depth of crevice corrosion of all the
alloys increased with depth, but not consistently. The
corrosion behavior of the 5000 Series aluminum
alloys appears to be more uniformly affected by
depth than by duration of exposure or changes in the
concentration of oxygen in seawater.

6.4.3. Effect of Concentration of Oxygen

The cor.'osion rates of alloy 5086-H34 increased
linearly with increasing concentration of oxygen in



188



seawater, but the slope of the line was very small (1
to 25). However, such relationships were not found
for the maximum depths of pitting and crevice cor-
rosion. The pit depths were a maximum at the highest
oxygen concentration, and the maximum depth of
crevice corrosion was at the intermediate oxygen con-
centration.

The corrosion rates of alloy 5456-H321 decreased
linearly with increasing concentration of oxygen in
seawater, but the slope of the line was very small (1
to 10). However, no correlations were possible
between maximum depth of pitting and crevice cor-
rosion.

The corrosion rates and changes in the maximum
depths of pits and crevice corrosion of the other 5000
Series aluminum alloys were erratic and inconsistent
with respect to changes in the concentration of
oxygen in seawater. Changes in the concentration of
oxygen in seawater did not exert a constant or uni-
form influence on the corrosion behavior of the 5000
Series aluminum alloys. This behavior, like that of the
stainless steels and some nickel alloys, can be
attributed to the dual role oxygen can play with
regard to alloys which depend upon passive films for
their corrosion resistance.



6.4.7. Corrosion Products

Corrosion products from alloy 5086 were
analyzed by X-ray diffraction, spectrographic
analysis, quantitative chemical analysis, and infra-red
spectrophotometry. The qualitative results were:
amorphous A1 2 3 -XH 2 0, NaCl, Si0 2 , Al, Na, Mg,
Cu, Fe, Si, Ti, 5.8% chloride ion, 26.2% sulfate ion,
and considerable phosphate ion.

6.4.8. Mechanical Properties

The effects of exposure on the mechanical pro-
perties of the 5000 Series aluminum alloys are given
in Table 72. The mechanical properties of the follow-
ing alloys were adversely affected by exposure:
5456-H321 after 123 days of exposure at the
6,000-foot depth; 5052-H32, 5083-H113, and
5456-H34 after 403 days of exposure at the
6,000-foot depth; and 5456-H321 and 5456-H34
after 751 days of exposure at the 6,000-foot depth.
The mechanical properties of the above alloys after
exposures for different times at different depths and
of the other alloys were not adversely affected by
exposure at depth in the seawater.



6.4.4. Stress Corrosion

Some 5000 Series aluminum alloys were exposed
at the depths and for the times given in Table 71
when stressed at values equivalent to 30, 50, or 75%
of their respective yield strengths to determine their
susceptibilities to stress corrosion. They were not sus-
ceptible to stress corrosion under the test conditions.

6.4.5. Other Types of Corrosion

Alloys 5052-H32 and 5456-H34 were attacked by
the exfoliation type of corrosion. Alloys 5083-H113,
5086-H32, and 5086-H34 were attacked by inter-
granular corrosion.

6.4.6. Welding

Welding did not affect the corrosion behavior of
alloys 5083-H113, 5086-H34, and 5454-H32.



6.5. 6000 SERIES ALUMINUM ALLOYS
(ALUMINUM-MAGNESIUM-SILICON ALLOYS)

The chemical compositions of the 6000 Series
aluminum alloys are given in Table 73, their corrosion
rates and types of corrosion in Table 74, their stress
corrosion behavior in Table 75, and the effects of
exposure on their mechanical properties in Table 76.

The aluminum-magnesium-silicon system is the
basis for a major class of heat-treatable aluminum-
base alloys. They combine many desirable characteris-
tics, including moderately high strength and good
resistance to corrosion.

There was only one 6000 Series alloy (6061) in
this program. Alloy 6061 corroded chiefly by the
crevice and pitting types of localized corrosion. Also,
there was some intergranular corrosion.

6.5.1. Duration of Exposure



The corrosion rates of 6061 at the surface and at
the 6,000-foot depth decreased with duration of



189



exposure, but not uniformly, while those at the
2,500-foot depth increased with duration of expo-
sure. However, the maximum depths of pitting and
crevice corrosion increased with increasing duration
of exposure at the surface and at depths of 2,500 and
6,000 feet.

6.5.2. Effect of Depth

Although the corrosion rates and the maximum
depths of pitting and crevice corrosion were greater at
depth than at the surface, these increases did not
increase uniformly with increasing depth. Depth
exerted no uniform influence on the corrosion
behavior of alloy 6061.

6.5.3. Effect of Concentration of Oxygen

The corrosion rates and maximum depths of
pitting and crevice corrosion decreased with
increasing concentration of oxygen in seawater. The
maximum depths of crevice corrosion decreased
linearly with increasing oxygen concentration. The
corrosion rates and maximum depths of pitting
decreased constantly, but not uniformly, with depth.

6.5.4. Stress Corrosion

Alloy 6061-T6 was exposed at the depths and for
the times given in Table 75 when stressed at values
equivalent to 30 and 75% of its yield strength to
determine its susceptibility to stress corrosion. Alloy
6061-T6 was not susceptible to stress corrosion under
the test conditions.

6.5.5. Welding

The corrosion of alloy 6061-T6 was adversely
affected by welding. Alloy 6061 was attacked by
intergranular corrosion in the "as-welded" condition.

6.5.6. Mechanical Properties

The effects of exposure on the mechanical pro-
perties of alloy 6061-T6 are given in Table 76. The
mechanical properties of 6061-T6 were adversely
affected by exposure in seawater. Those specimens
which had been welded and which had been attacked
by intergranular corrosion were the most seriously
affected.



6.6. 7000 SERIES ALUMINUM ALLOYS
(ALUMINUM-ZINC-MAGNESIUM ALLOYS)

The chemical compositions of the 7000 Series
aluminum alloys are given in Table 77, their corrosion
rates and types of corrosion in Table 78, their stress
corrosion behavior in Table 79, and the effect of
exposure on their mechanical properties in Table 80.

Combinations of zinc and magnesium in
aluminum provide a class of heat-treatable alloys,
some of which develop the highest strengths presently
known for commercial aluminum-base alloys. The
addition of copper to the aluminum-zinc-magnesium
system, together with small but important amounts
of chromium and manganese, results in the highest
strength, heat-treatable, aluminum-base alloys
commercially available.

The 7000 Series alloys were attacked by crevice,
edge, exfoliation, intergranular, and pitting types of
corrosion. Corrosion of the Alclad alloys was by
shallow pitting and crevice corrosion, slight blistering,
and general corrosion.

Because of the erratic behavior of the 7000 Series
aluminum alloys during exposure in seawater at
depth, it was impossible to find any correlation
between their corrosion behavior and duration of
exposure, effect of depth, or the effect of changes in
the concentration of oxygen in seawater.

A practical case of unusual corrosion on an
aluminum alloy was encountered with the Alclad
7178-T6 aluminum alloy buoys used in the installa-
tion of the STU structures. During the retrieval of
STU 1-3 after 123 days of exposure, the buoy, which
was 300 feet below the surface, was found to be
corroded. White corrosion products on the bottom
hemisphere covered areas where the cladding alloy
had corroded through to the core material. The top
hemisphere was blistered, the blisters being as large as
2 inches in diameter and 0.75 inch high with a hole in
the top of each blister. The hole in the top of the
blister indicates the origin of the failure: originally a
pinhole in the cladding alloy existed where seawater
gained access to the interface between the cladding
alloy and the core alloy. When this blister was
sectioned to inspect the corrosion underneath, it was
found to be filled with white crystalline aluminum
oxide corrosion products. It appeared that seawater
penetrated the cladding alloy at a defect, or a pit was
initiated at a particle of a cathodic metal (probably



190



iron), and the corrosion was then concentrated at the
interface between the two alloys (cladding alloy and
core alloy). The thickness of the remaining Alclad
layer indicated that it had not been sacrificed to
protect the core alloy as was its intended function.
On the other hand, the selective corrosion of the
Alclad layer on the bottom hemisphere and the
uncorroded core material showed that, in this case,
the cladding alloy was being sacrificed to protect the
core material as intended.

When an attempt was made to repair these buoys
for reuse by grinding off all traces of corrosion prior
to painting, it was found that the corrosion had pene-
trated along the interface between the cladding alloy
and the core alloy for considerable distances from the
edges of the blisters and the edges of the holes where
the cladding alloy layer had been sacrificed. Polished
transverse sections taken from the buoy through
these corroded areas corroborated the indications
found from grinding operations. Metallurgical
examinations showed that the corroded paths were,
in fact, entirely in the cladding alloy, with a thin
diffusion layer of material between the corrosion
path and the core material.

Blistering of Alclad aluminum alloys such as
encountered with these Alclad 7178-T6 spheres was
very unusual. Blistering due to corrosion and the
rapid rate of sacrifice of Alclad layers had not been
encountered previously by the author and other
investigators in surface seawater applications. Because
of this unique blistering one of the spheres was sent
to the Research Laboratories of the Aluminum Com-
pany of America where an investigation was made to
determine the mechanism of this behavior.

Wei [15] showed that there was preferential
diffusion of zinc over copper from the core alloy into
this interfacial zone. The high zinc and low copper
contents of this interfacial zone rendered it anodic to
both the cladding and core alloys. Selective attack
was inevitable once corrosion reached this anodic
diffusion zone.

That this type of blistering has been encountered
on buoys at depths from 300 to 6,800 feet
emphasizes the fact that there is some factor present
which either is more influential at depth or is not
present at the surface. The fact that this thin anodic
zone is probably present in all Alclad 7178-T6 pro-
ducts and, as such, is not blistered during surface



seawater exposures indicates that the seawater
environments at depths of 300 feet and greater differ
from the seawater environments at the surface, at
least with respect to the corrosion behavior of this
alloy.

6.6.1. Stress Corrosion

The 7000 Series aluminum alloys were exposed at
the depths and for the times given in Table 79 when
stressed at values equivalent to 30, 50, and 75% of
their respective yield strengths to determine their sus-
ceptibilities to stress corrosion. Alloys 7075-T6,
7079-T6, Alclad 7079-T6, and 7178-T6, failed by
stress corrosion cracking.

6.6.2. Corrosion Products

Corrosion products from alloy 7079-T6 were

analyzed by X-ray diffraction, spectographic analysis,
quantitative chemical analysis, and infra-red spectro-
photometry. The qualitative results were: amor-
phorous A1 2 3 "XH 2 0, NaCl, Al metal, Al, Cu, Mg,
Mn, Zn, Na, Ca, traces of Ti and Ni, 2.82% chloride
ion, 16.7% sulfate ion, and considerable phosphate
ion.

6.6.3. Mechanical Properties

The effects of exposure on the mechanical pro-
perties of the 7000 Series aluminum alloys are given
in Table 80. The mechanical properties of alloys
7002-T6, 7039-T6, 7075-T6, 7075-T64, 7075-T73,
7079-T6, and 7178-T6 were adversely affected.



191



Table 57. Chemical Composition of 1000 Series Aluminum Alloys, Percent by Weight



Alloy


Gage
(in.)


Si


Fe


Cu


Mn


Zn


Al fl


1100


-


-





-





-


99.0


1100-0


-


b


b


0.14


0.03


-


R


1100-H14


0.050


0.14


0.55


0.14


-


0.06


R


1180


0.050


0.06


0.08


0.002


0.002


-


R



fl R = remainder.
fc Si+ Fe = 0.57



Table 58. Corrosion Rates and Types of Corrosion of 1000 Series Aluminum Alloys



Alloy


Environment


Exposure
(day)


Depth
(ft)




C


orrosion




Source


Rate
(mpy)


Maximum

Pit Depth

(mils)


Crevice
Depth*

(mils)


_, b

Type


1100-H14


W


123


5,640


2.0


39





P


INCO(3)


1100-H14


S


123


5,640


3.1


41





P


INCO(3)


1100-H14


W


403


6,780


4.2





62


C(PR)


INCO(3)


110OH14


S


403


6,780


1.3


62


62


C (PR);P (PR)


INCO (3)


1100-H14


W


751


5,640


4.5





62


C(PR)


INCO(3)


1100-H14


S


751


5,640


2.0





62


C(PR)


INCO (3)


110OH14


W


1,064


5,300


3.0





62


C(PR)


INCO (3)


1100-H14


W


1,064


5,300


1.8


S-P


S-C


S-C ; S-P


CEL (4)


1100-H14


S


1,064


5,300


1.0





62


C(PR)


INCO (3)


1100-H14


W


197


2,340


5.6





62


C(PR)


INCO (3)


1100-H14


w


197


2,340


<0.1


23





E;P


REY(14)


1100-H14


s


197


2,340


<0.1


I


1


I-C;I-P


INCO (3)


1100-H14


w


402


2,340


1.6





62


C(PR)


INCO (3)


1100-H14


w


402


2,370


0.9


I


50


C(PR);I-P


CEL (4)


1100-H14


s


402


2,370


0.5


26


26


C;P


INCO (3)


1100-H14


s


402


2,370


0.5


I


50


C (PR); IP


CEL (4)


1100-H14


w


181


5


1.4





S


S-C


INCO (3)


1100-H14


w


366


5


0.6


13


13


C;P


INCO (3)


1180-H14


w


402


2,370


1.0


1


50


C(PR);I-P


CEL (4)


1180


w


402


2,370


<0.1


41





E;S-P


REY(14)


1180-H14


s


402


2,370


0.8


I


50


C (PR); IP


CEL (4)



W = Totally exposed in seawater on sides of structure; S = Exposed in base of structure so that the lower portions of
the specimen were embedded in the bottom sediments.



Symbols for types of corrosion:






C = Crevice


P


= Pitting


E = Edge


PR


= Perforated


I = Incipient


S


= Severe



"Numbers refer to references at end of report.



192



Table 59. Stress Corrosion of 1000 Series Aluminum Alloys



Alloy


Stress
(ksi)


Percent

Yield

Strength


Exposure

(day)


Depth
(ft)


Number of

Specimens

Exposed


Number
Failed


Source' 7


1100-H14
1100-H14

1180
1180


8

12

6
10


50

75

50

75


402
402

402
402


2,370
2,370

2,370
2,370


3
3

3
3









CEL (4)
CEL (4)

CEL (4)
CEL (4)






"Numbers refer to references at end of report.



Table 60. Changes in Mechanical Properties of 1000 Series Aluminum Alloys Due to Corrosion



Exposure Depth
Alloy ;. , ,/.
' (day) (ft)


Tensile Strength


Yield Strength


Elongation


Source


Original

(ksi)


% Change


Original
(ksi)


% Change


Original
(%)


% Change


1100-H14 402 2,370
1180 402 2,370


19
15


-2
+ 1


18

14


+ 3
+4


7
11


+ 14
+ 1


CEL (4)
CEL (4)



Numbers refer to references at end of report.



Table 61. Chemical Composition of 2000 Series Aluminum Alloys, Percent by Weight



Alloy


Gage

(in.)


Si


Fe


Cu


Mn


Mg


Cr


Ni


Zn


Ti


Other


Al°


2014-Te


_


0.80


0.34


3.90


0.70


0.50


0.01


0.01


0.03


-


-


R


2014-T6


0.050


0.91


0.53


4.23


0.80


0.32


0.03


0.01


0.08


0.02


-


R


2014-T6


-


0.84


0.35


4.43


0.80


0.66


-


-


-


-


-


R


2024


0.064


-


-


4.3


0.6


1.5


-


-


-


-




R


2024-T3+T81


-


0.20


0.20


4.50


0.80


1.50


0.02


0.04


0.06


0.01




R


Alclad 2024-T3*


























2219-T81


0.064


0.20


0.30


6.3


0.30


<0.02


-


-


0.10


0.06


0.10 V
0.17 Zr


R


2219-T81


-


0.05


0.15


4.00


0.10


0.05


0.05


0.10


0.05


0.05


-


R


2219-T87


0.040


0.08


0.12


6.54


0.26


<0.02


<0.02


<0.02


0.03


0.02


0.10 V
0.15 Zr


R


2219-T87


0.040


-


-


6.3


0.30




"


"


-


0.06


-


R



R = remainder.
No analysis given.



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Online LibraryFrederick Morris ReinhartCorrosion of metals and alloys in the deep ocean → online text (page 20 of 27)