The Sub Marine Explorer was designed and built by Julius H. Kroehl, who was born in Prussia in 1820. After migrating to the United States in 1838 and becoming a citizen, he served in the Union Navy (United States Navy during the Civil War) as an underwater demolitions expert. He left the Navy in 1863 and began designing a “sub-marine” that would facilitate Union forces’ mine removal and obstruction clearance. At the end of the Civil War, he became an engineer for the Pacific Pearl Co., an organization interested in using the craft to recover pearls from deep sea oyster beds in the Bay of Panama. Decompression sickness (the bends), unknown at the time, began to affect the crew in 1869, which led to the abandonment of Explorer in the tidal zone of St. Elmo’s Island (Isla San Telmo) in the Archipielago de las Perlas, Panama

Carr, James D.

Delgado, James P.

Johnson, Donald L.

Murphy, Larry E.

Wilson, Brent M.

English

UNL | Libraries

University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
Faculty Publications -- Chemistry Department Published Research - Department of Chemistry
2010
Corrosion of Civil War Era Sub Marine
Explorer—Part 2
Donald L. Johnson
Submerged Resources Center, National Park Service, johnsondonl@aol.com
James D. Carr
University of Nebraska - Lincoln, jcarr1@unl.edu
Brent M. Wilson
Amsted Rail, bwilson@amstedrail.com
Larry E. Murphy
Submerged Cultural Resource Consultants, lmurphy56@comcast.net
James P. Delgado
Texas A&M University, jdelgado@inadiscover.com
Follow this and additional works at: https://digitalcommons.unl.edu/chemfacpub
Part of the Analytical Chemistry Commons, Medicinal-Pharmaceutical Chemistry Commons,
and the Other Chemistry Commons
This Article is brought to you for free and open access by the Published Research - Department of Chemistry at DigitalCommons@University of
Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications -- Chemistry Department by an authorized administrator of
DigitalCommons@University of Nebraska - Lincoln.
Johnson, Donald L.; Carr, James D.; Wilson, Brent M.; Murphy, Larry E.; and Delgado, James P., "Corrosion of Civil War Era Sub
Marine Explorer—Part 2" (2010). Faculty Publications -- Chemistry Department. 193.
https://digitalcommons.unl.edu/chemfacpub/193
50  MATERIALS PERFORMANCE  October 2010 NACE International, Vol. 49, No. 10
Continued on page @@
Corrosion of 
Civil War Era 
Sub Marine 
Explorer—Part 2 
DonalD l. Johnson anD James D. Carr, 
University of Nebraska-Lincoln, Lincoln, Nebraska
Brent m. Wilson, Amsted Rail, Granite City, Illinois
larry e. murphy, Submerged Cultural Resource Consultants,
Panama City, Florida
James p. DelgaDo, Institute of Nautical Archaeology, 
Texas A&M University, College Station, Texas 
This is Part 2 of a two-part series on the  
Sub Marine Explorer. Part 1 was published in the 
September 2010 issue of  MP. This article focuses 
on the experimental results obtained from  
the corrosion assessment of the American Civil  
War-era craft’s hull. 
T
he Sub Marine Explorer was de-
signed and built by Julius H. 
Kroehl, who was born in Prussia 
in 1820. After migrating to the 
United States in 1838 and becoming a 
citizen, he served in the Union Navy 
(United States Navy during the Civil 
War) as an underwater demolitions ex-
pert. He left the Navy in 1863 and began 
designing a “sub-marine” that would 
facilitate Union forces’ mine removal and 
obstruction clearance. At the end of the 
Civil War, he became an engineer for the 
Pacific Pearl Co., an organization inter-
ested in using the craft to recover pearls 
from deep sea oyster beds in the Bay of 
Panama. Decompression sickness (the 
bends), unknown at the time,  began to 
affect the crew in 1869, which led to the 
abandonment of Explorer in the tidal zone 
of St. Elmo’s Island (Isla San Telmo) in 
the Archipielago de las Perlas, Panama. 
The original structural features of Sub 
Marine Explorer, discussed in Part I, have 
been compromised from over 140 years 
of exposure in the tidal zone. Drawings 
by John W. McKay (Figure 1) illustrate 
major hull damage, on both the port 
and starboard sides, to the wrought 
iron above the hull midline and cast iron 
below it. 
Experimental Procedure 
and Results
Corrosion Potential 
Measurements
As part of the initial analysis, corrosion 
potential measurements were taken on 
the exterior hull and in the interior work-
ing chamber. A pneumatic drill and flat 
mill bit were used to remove oxide and 
concretion to base metal on both sides of 
the hull above the midline. After swaging 
a copper ground wire into a hole drilled 
through the shell plate, the contact area 
was sealed with waterproof epoxy. A sil-
ver-silver chloride (Ag/AgCl) seawater 
Continued on page @@
NACE International, Vol. 49, No. 10  October 2010 MATERIALS PERFORMANCE  51
M AT E R I A L S  S E L E C T I O N  &  D E S I G N
equivalent, GMC Electrical† reference 
electrode (+245 mV conversion to hydro-
gen electrode) was positioned on transects 
by a diver directed via hard-wire com-
munication from shore. The corrosion 
potential, E
corr
, data were recorded with 
an HD-160B Wavetech† high-resistance 
voltmeter. The probe was returned to 
GMC Electrical after the field operation 
for calibration verification and was found 
to have remained within tolerance. 
External Hull Horizontal Transects
Six duplicate horizontal transects were 
obtained from the starboard side and five 
single transects from the port side. A 
typical transect is shown in Figure 2. 
The corrosion potential, E
corr
, which is 
representative of corrosion rate, decreases 
from bow to stern, as is expected based 
on previous analysis.1 The same trend 
shown in Figure 2 appears seaside (port), 
although E
corr
 is generally higher than 
either transect on the starboard side.
External Hull Vertical Transect
All transects were taken horizontally. 
To determine the effect of tide depth, 
however, the data were transposed to the 
vertical. Five vertical starboard transects 
and five port were calculated at horizon-
tal positions from bow to stern. With one 
exception, all transects indicate E
corr
 de-
creases from the hull top to the sand line.
Interior Working Chamber
Four transects were obtained inside 
the working chamber on the top center-
line, port, starboard, and bottom. Be-
cause of heavy concretion, E
corr
 values 
were generally lower by 20 to 30 mV, 
indicating lower corrosion rates. All tran-
sects in the working chamber consistently 
show increasing E
corr
 from bow to stern, 
the reversal reflecting water flow from 
bow to stern inside the working chamber. 
Environmental Monitoring
A YSI 6-series Sonde† sensor was in-
stalled at interior and exterior positions 
to continuously monitor pH, tempera-
ture, salinity, and dissolved oxygen (DO) 
in the water. Data were obtained over a 
four-day period. The maximum re-
corded pH was 8.3 and the minimum 
was 8.1. With two exceptions, DO was 
tracked high to low tide with a recorded 
maximum of 9.1 mg/L and minimum 
6.6 mg/L for an average of 7.7 mg/L. 
The highest temperature recorded was 
22.2 °C and the lowest was 18.8 °C for 
an average of 20.5 °C.
†Trade name.
Sub Marine Explorer (1865) external hull damage depicted as of February 2008. 
(a) Port side and (b) starboard side. Drawings by John W. McKay, March 2008.
Ecorr (mV, Ag/AgCl seawater equivalent) transect 20.1/22.2 starboard, horizontal 
sand line as a function of distance from bow section “Om,” cast iron (Ecorr = 
–0.124x – 477.4, R2 = 0.323).
FIguRE 1
FIguRE 2
52  MATERIALS PERFORMANCE  October 2010 NACE International, Vol. 49, No. 10
Continued on page @@
M AT E R I A L S  S E L E C T I O N  &  D E S I G N Corrosion of Civil War Era Sub Marine Explorer—Part 2
Direct Hull Thickness 
Measurement
An area of the hull adjacent to the 
ground connection was chosen to measure 
metal thickness. Shiny base metal ap-
peared after removing 42 mm (1.65 in) of 
scale from the exterior side. Using a hole 
drilled through the hull, caliper readings 
were made and averaged to yield a metal 
thickness of 12.2 mm (0.48 in). Assuming 
the hole was drilled in an overlap area, 
one-half of the original wall thickness re-
mains, which corresponds to an average 
corrosion rate of 12.2/137 = 0.089 mmpy 
(3.5 mpy) or 0.044 mmpy (1.8 mpy) per 
side. This corrosion rate is somewhat 
higher than the reported 0.036 mmpy 
(1.4 mpy) for exposure in the tidal zone 
but lower than that reported for continu-
ous immersion as noted in Part 1.2    
Sample Collection and  
Data Acquisition
X-ray diffraction scans of concretion 
samples revealed magnetite (Fe
3
O
4
), ma-
ghemite (a-Fe
2
O
3
), and siderite (FeCO
3
) 
as major constituents. Unlike concretion 
samples from the USS Arizona, aragonite 
(CaCO
3
) was not detected. The x-ray 
intensity of siderite was relatively low 
compared to USS Arizona concretion 
because of dilution of the concretion with 
oxide corrosion products and lower cal-
cium content. Iron exchange with cal-
cium in CaCO
3
 was consequently low. 
Maghemite is slightly magnetic.
Concretion Equivalent  
Corrosion Rate
The first two columns in Table 1 
identify sample number, location, and 
metal type. The third column tabulates 
density using the submergence method 
described in ASTM D792-00.3 Concre-
tion thickness, iron content, and corro-
sion rate are tabulated in subsequent 
columns. Equation (1) was used to deter-
mine corrosion rates with the exception 
of Sample 06-007 as noted in the com-
ments column: 
 i
corr
 = 0.8 ρd(wt%Fe)/t mpy (1) 
where ρ = concretion density (gm/cm3), 
d = concretion thickness (cm), wt%Fe = 
wt% of iron (dry basis), and t = exposure 
time (years).
Concretion samples on the Explorer are 
not totally remnants of marine organisms 
as is typically the case in quiet seawater, 
but include imbedded oxide products as 
previously mentioned. 
Available Oxygen Corrosion Rate
As a diagnostic tool to provide infor-
mation regarding rate control, the appar-
ent oxygen corrosion rate, i
aocr
, is calcu-
lated from Fick’s First Law according to 
Equation (2):
 i
aocr
 = K nFC(O
2
)D(O
2
)/d (mpy) (2) 
where K = 5550 (conversion constant), 
n = number of equivalents per gram 
atomic weight, F = Faraday constant 
(96,500 A-s/equivalent), C(O
2
) = oxygen 
concentration (mg/L), D(O
2
) = oxygen 
diffusion coefficient (cm2/s), and d = dif-
fusion thickness (cm).
Using D(O
2
) = 2.24  10–5, C(O
2
) = 
7.7, and d(ave) = 1.37, collecting terms 
and constants, Equation (2) simplifies to: 
i
aocr
 = 5550 [(2.24  10–5] 7.7/1.36 ≈ 
 0.7 mpy (0.0178 mmpy) (3) 
The i
corr
 is substantially higher than i
aocr
 
by the ratio i
corr
/i
aocr
 = 1.2/0.7 = 1.7, in-
dicating that cathodic hydrogen dis-
charge supports the corrosion process.4 
Assuming the corrosion rate of com-
pletely submerged wrought iron is 70% 
higher,1 the ratio increases to (1.2  0.7 
+ 1.2)/0.7 ≈ 2.86. This ratio (actual cor-
rosion rate/corrosion rate controlled by 
oxygen consumption) is defined as the 
Weins Number, Wn.4 
Discussion
Because the port side (seaside) under-
goes more intermittent splash than the 
starboard side, high corrosion rates are 
primarily responsible for the disappear-
ance of hull metal on that side. Mechan-
ical impact from wave-borne floating 
debris and abrasion are contributing fac-
tors. The carryover appears similar to 
TAbLE 1
Concretion sample location, properties, and corrosion rate
Sample
Location 
(Metal)
ρ
(g/cm3)
d
(cm)
Fe
wt%
CECR
(mpy) Comments
06-002 Low stern
(cast)
2.24 0.98 57.59 0.8 CECR, exterior, 
broken, FeCO3
06-003 Midship
(wrought)
2.28 1.75 60.98 1.4 CECR, interior,  
chiseled,  
0.1 wt% Ca
06-005 Aft, tower
(wrought)
2.30 0.80 48.01 0.5 CECR, interior, 
drill/saw,  
4.3 wt% Ca
06-006 Aft
(cast)
2.40 2.36 53.7 1.8 CECR, exterior, 
drill/saw,  
0.3 wt% Ca
06-007 Midship
(wrought)
— — — 1.8 Calipers, base 
metal
06-018 Midship
(wrought)
3.01 0.95 48.94 0.8 CECR, interior, 
chisled
16 wt% Ca
Average 1.2 0.03 mmpy
ρ = concretion density (ASTM D792-00), d = thickness, Fe wt% = iron content, and CECR = 
concretion equivalent corrosion rate.
Continued on page @@
NACE International, Vol. 49, No. 10  October 2010 MATERIALS PERFORMANCE  53
M AT E R I A L S  S E L E C T I O N  &  D E S I G N
flash corrosion wherein high oxygen 
leve l s  y ie ld  ferr ic  oxyhydroxide 
[FeO(OH)]. This corrosion product tends 
to dry out to form hematite and magnetic 
maghemite, the reddish-orange to red 
ferric oxides. Evidence of these oxides 
was observed in various forms depending 
upon the degree of dryness on the upper 
starboard hull. 
Conclusions
The reported results are consistent 
with U.S. Navy corrosion data. The up-
per limit to the deterioration rate of 
Sub Marine Explorer is unknown because 
of the extent of mechanical impact and 
abrasion. As discussed, the corrosion 
rate is variable depending upon pro-
tection against corrosion that is pro-
vided by concretion and the position of 
various parts of the hull in relation to 
tide flow pattern and flash corrosion 
from carryover. 
An average corrosion rate of 0.03 
mmpy (1.2 mpy) is estimated for the ex-
terior hull side. Assuming the same cor-
rosion rate on the interior hull side, the 
average corrosion rate for both sides is 
nearly 0.061 mmpy (2.4 mpy). Based on 
data and observations on the upper star-
board side, about one-half of the original 
thickness of the upper outer hull remains; 
structural integrity of this part of the hull 
structure is reaching critical stage. Cor-
rosion rate control involves the interac-
tion of oxygen consumption and hydro-
gen discharge with the latter apparently 
being dominant. 
The authors recommend that cast and 
wrought-iron samples be obtained from 
the site for detailed metallographic ex-
amination to: 1) confirm the assumption 
that the cast portion of Explorer is gray cast 
iron in terms of today’s classification, and 
2) determine corrosion rate from the 
depth of the graphitized zone in the cast 
iron microstructure for comparison with 
results in Table 1.
Acknowledgments
The analysis of Sub Marine Explorer was 
accomplished under permit by the Gov-
ernment of Panama through the Instituto 
Nacional De Cultura in two separate field 
sessions in 2004 and 2006. We also ac-
knowledge the financial support of the 
Council of American Maritime Museums 
(CAMM) and the Office of Ocean Explo-
ration of the National Oceanic and At-
mospheric Administration (NOAA). A 
subsequent field season completed docu-
mentation of the craft in 2008 and was 
funded by Waitt Institute for Discovery.
Archaeological survey team members 
in 2006 were James Delgado, Todd Cro-
teau, Bert Ho, Michael McCarthy, Larry 
Murphy, Joshua Price, Don Johnson, and 
Karl Vandenhole. The authors also thank 
the NACE International Omaha Section 
for providing undergraduate scholarship 
funding for student laboratory assistance 
at the University of Nebraska-Lincoln, 
and the National Park Service Sub-
merged Resources Center for its support. 
References
1 I.D. McCleod, C.V. Ruppe, J.F. Barstad, 
eds., “In Situ Corrosion Measurements 
and Management of Shipwreck Sites,” 
Intl. Handbook of Underwater Archeology, pp. 
697-714.
2 M. Schumacher, ed., Seawater Corrosion 
Handbook (Park Ridge, NJ: Noyes Data 
Corp., 1979), pp. 13-14; 367-387. 
3 ASTM D792-00, “Standard Test  
Methods for Density and Specific  
Gravity (Relative Density) of Plastics by 
Displacement” (West Conshohocken, 
PA: ASTM Standards, 2000).
4 D.L. Johnson, D.J. Medlin, D.L. Conlin, 
L.E. Murphy, M.A. Russell, J.D. Carr, 
B.M. Wilson, “The Weins Number— 
Integrated Long-Term Corrosion Decay 
Trajectories of Iron-Based Alloy Ship-
wrecks and Artifacts on the Sea Floor,” 
Research in Progress (RIP) Symposium 
Extended Abstracts, March 14-18, 2010, 
NACE CORROSION 2010, San  
Antonio, Texas.
Editor’s note: Sub Marine Explorer was recently 
featured in the Smithsonian Network’s “America’s Lost 
Submarine” and Speigel TV International’s “The 
Search for Explorer,” History U.K. and U.S. versions.
DONALD L. JOhNSON is a volunteer metallurgical 
engineer/corrosion scientist at the Submerged 
Resources Center, National Park Service, 14709 W. 
Via Manana, Sun City West, AZ 85375, e-mail: 
johnsondonl@aol.com. he is also a professor 
emeritus at the University of Nebraska-Lincoln.  
his recent experience includes corrosion of 
submerged resources; past experience includes 
teaching and research with publications in the 
areas of thermodynamics, hydrogen transport, and 
corrosion. he was lead engineer in the develop-
ment of the Concretion Equivalent Corrosion Rate 
(CECR) concept pioneered during the USS Arizona 
Preservation Project. he was awarded the George 
B. hartzog National Park Service Volunteer of the 
Year Award in 2006. he is a 44-year member of 
NACE International.
LARRY E. MURPhY is an archaeologist with 
Submerged Cultural Resource Consultants, PO Box 
15065, Panama City, FL 32406, e-mail: lmurphy56@
comcast.net. he previously was chief of the 
National Park Service Submerged Resource Center 
for 30 years, during which time he participated or 
led more than 100 underwater archeological 
research projects. he has an M.A. degree and 
received the Society for historical Archaeology 
Award of Merit.
JAMES D. CARR is a professor emeritus of 
chemistry at the University of Nebraska-Lincoln, 
Department of Chemistry, Lincoln, NE 68588-0304, 
e-mail: jcarr1@unl.edu. Prior to retirement, he 
taught chemistry and directed graduate research in 
analytical chemistry. he has a Ph.D. in analytical 
chemistry from Purdue University and authored the 
textbook, Analytical Chemistry and Quantitative 
Analysis.
BRENT WILSON is director of Research and 
Development at Amsted Rail, 1700 Walnut St.,  
Granite City, IL 62040, e-mail: bwilson@amstedrail.
com. he formerly was a professor at the University 
of Nebraska-Lincoln. he has a Ph.D. in chemical and 
materials engineering and is an eight-year member 
of NACE.
JAMES P. DELGADO is president and chief operating 
officer at the Institute of Nautical Archaeology,  
PO Drawer hG, College Station, TX 77841, e-mail: 
jdelgado@inadiscover.com. he is a maritime 
archaeologist who specializes in 19th century 
wrecks. he was the principal investigator for the 
Sub Marine Explorer archaeological missions and 
documentation of the site. 
View publication stats


Corrosion of
Civil War Era
\u3ci\u3eSub Marine
Explorer\u3c/i\u3e—Part 2

DigitalCommons@University of Nebraska

University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnFaculty Publications -- Chemistry Department Published Research - Department of Chemistry2010Corrosion of Civil War Era Sub MarineExplorer—Part 2Donald L. JohnsonSubmerged Resources Center, National Park Service, johnsondonl@aol.comJames D. CarrUniversity of Nebraska - Lincoln, jcarr1@unl.eduBrent M. WilsonAmsted Rail, bwilson@amstedrail.comLarry E. MurphySubmerged Cultural Resource Consultants, lmurphy56@comcast.netJames P. DelgadoTexas A&M University, jdelgado@inadiscover.comFollow this and additional works at: https://digitalcommons.unl.edu/chemfacpubPart of the Analytical Chemistry Commons, Medicinal-Pharmaceutical Chemistry Commons,and the Other Chemistry CommonsThis Article is brought to you for free and open access by the Published Research - Department of Chemistry at DigitalCommons@University ofNebraska - Lincoln. It has been accepted for inclusion in Faculty Publications -- Chemistry Department by an authorized administrator ofDigitalCommons@University of Nebraska - Lincoln.Johnson, Donald L.; Carr, James D.; Wilson, Brent M.; Murphy, Larry E.; and Delgado, James P., "Corrosion of Civil War Era SubMarine Explorer—Part 2" (2010). Faculty Publications -- Chemistry Department. 193.https://digitalcommons.unl.edu/chemfacpub/19350  MATERIALS PERFORMANCE  October 2010 NACE International, Vol. 49, No. 10Continued on page @@Corrosion of Civil War Era Sub Marine Explorer—Part 2 DonalD l. Johnson anD James D. Carr, University of Nebraska-Lincoln, Lincoln, NebraskaBrent m. Wilson, Amsted Rail, Granite City, Illinoislarry e. murphy, Submerged Cultural Resource Consultants,Panama City, FloridaJames p. DelgaDo, Institute of Nautical Archaeology, Texas A&M University, College Station, Texas This is Part 2 of a two-part series on the  Sub Marine Explorer. Part 1 was published in the September 2010 issue of  MP. This article focuses on the experimental results obtained from  the corrosion assessment of the American Civil  War-era craft’s hull. The Sub Marine Explorer was de-signed and built by Julius H. Kroehl, who was born in Prussia in 1820. After migrating to the United States in 1838 and becoming a citizen, he served in the Union Navy (United States Navy during the Civil War) as an underwater demolitions ex-pert. He left the Navy in 1863 and began designing a “sub-marine” that would facilitate Union forces’ mine removal and obstruction clearance. At the end of the Civil War, he became an engineer for the Pacific Pearl Co., an organization inter-ested in using the craft to recover pearls from deep sea oyster beds in the Bay of Panama. Decompression sickness (the bends), unknown at the time,  began to affect the crew in 1869, which led to the abandonment of Explorer in the tidal zone of St. Elmo’s Island (Isla San Telmo) in the Archipielago de las Perlas, Panama. The original structural features of Sub Marine Explorer, discussed in Part I, have been compromised from over 140 years of exposure in the tidal zone. Drawings by John W. McKay (Figure 1) illustrate major hull damage, on both the port and starboard sides, to the wrought iron above the hull midline and cast iron below it. Experimental Procedure and ResultsCorrosion Potential MeasurementsAs part of the initial analysis, corrosion potential measurements were taken on the exterior hull and in the interior work-ing chamber. A pneumatic drill and flat mill bit were used to remove oxide and concretion to base metal on both sides of the hull above the midline. After swaging a copper ground wire into a hole drilled through the shell plate, the contact area was sealed with waterproof epoxy. A sil-ver-silver chloride (Ag/AgCl) seawater Continued on page @@NACE International, Vol. 49, No. 10  October 2010 MATERIALS PERFORMANCE  51M AT E R I A L S  S E L E C T I O N  &  D E S I G Nequivalent, GMC Electrical† reference electrode (+245 mV conversion to hydro-gen electrode) was positioned on transects by a diver directed via hard-wire com-munication from shore. The corrosion potential, Ecorr, data were recorded with an HD-160B Wavetech† high-resistance voltmeter. The probe was returned to GMC Electrical after the field operation for calibration verification and was found to have remained within tolerance. External Hull Horizontal TransectsSix duplicate horizontal transects were obtained from the starboard side and five single transects from the port side. A typical transect is shown in Figure 2. The corrosion potential, Ecorr, which is representative of corrosion rate, decreases from bow to stern, as is expected based on previous analysis.1 The same trend shown in Figure 2 appears seaside (port), although Ecorr is generally higher than either transect on the starboard side.External Hull Vertical TransectAll transects were taken horizontally. To determine the effect of tide depth, however, the data were transposed to the vertical. Five vertical starboard transects and five port were calculated at horizon-tal positions from bow to stern. With one exception, all transects indicate Ecorr de-creases from the hull top to the sand line.Interior Working ChamberFour transects were obtained inside the working chamber on the top center-line, port, starboard, and bottom. Be-cause of heavy concretion, Ecorr values were generally lower by 20 to 30 mV, indicating lower corrosion rates. All tran-sects in the working chamber consistently show increasing Ecorr from bow to stern, the reversal reflecting water flow from bow to stern inside the working chamber. Environmental MonitoringA YSI 6-series Sonde† sensor was in-stalled at interior and exterior positions to continuously monitor pH, tempera-ture, salinity, and dissolved oxygen (DO) in the water. Data were obtained over a four-day period. The maximum re-corded pH was 8.3 and the minimum was 8.1. With two exceptions, DO was tracked high to low tide with a recorded maximum of 9.1 mg/L and minimum 6.6 mg/L for an average of 7.7 mg/L. The highest temperature recorded was 22.2 °C and the lowest was 18.8 °C for an average of 20.5 °C.†Trade name.Sub Marine Explorer (1865) external hull damage depicted as of February 2008. (a) Port side and (b) starboard side. Drawings by John W. McKay, March 2008.Ecorr (mV, Ag/AgCl seawater equivalent) transect 20.1/22.2 starboard, horizontal sand line as a function of distance from bow section “Om,” cast iron (Ecorr = –0.124x – 477.4, R2 = 0.323).FIguRE 1FIguRE 252  MATERIALS PERFORMANCE  October 2010 NACE International, Vol. 49, No. 10Continued on page @@M AT E R I A L S  S E L E C T I O N  &  D E S I G N Corrosion of Civil War Era Sub Marine Explorer—Part 2Direct Hull Thickness MeasurementAn area of the hull adjacent to the ground connection was chosen to measure metal thickness. Shiny base metal ap-peared after removing 42 mm (1.65 in) of scale from the exterior side. Using a hole drilled through the hull, caliper readings were made and averaged to yield a metal thickness of 12.2 mm (0.48 in). Assuming the hole was drilled in an overlap area, one-half of the original wall thickness re-mains, which corresponds to an average corrosion rate of 12.2/137 = 0.089 mmpy (3.5 mpy) or 0.044 mmpy (1.8 mpy) per side. This corrosion rate is somewhat higher than the reported 0.036 mmpy (1.4 mpy) for exposure in the tidal zone but lower than that reported for continu-ous immersion as noted in Part 1.2    Sample Collection and  Data AcquisitionX-ray diffraction scans of concretion samples revealed magnetite (Fe3O4), ma-ghemite (a-Fe2O3), and siderite (FeCO3) as major constituents. Unlike concretion samples from the USS Arizona, aragonite (CaCO3) was not detected. The x-ray intensity of siderite was relatively low compared to USS Arizona concretion because of dilution of the concretion with oxide corrosion products and lower cal-cium content. Iron exchange with cal-cium in CaCO3 was consequently low. Maghemite is slightly magnetic.Concretion Equivalent  Corrosion RateThe first two columns in Table 1 identify sample number, location, and metal type. The third column tabulates density using the submergence method described in ASTM D792-00.3 Concre-tion thickness, iron content, and corro-sion rate are tabulated in subsequent columns. Equation (1) was used to deter-mine corrosion rates with the exception of Sample 06-007 as noted in the com-ments column:  icorr = 0.8 ρd(wt%Fe)/t mpy (1) where ρ = concretion density (gm/cm3), d = concretion thickness (cm), wt%Fe = wt% of iron (dry basis), and t = exposure time (years).Concretion samples on the Explorer are not totally remnants of marine organisms as is typically the case in quiet seawater, but include imbedded oxide products as previously mentioned. Available Oxygen Corrosion RateAs a diagnostic tool to provide infor-mation regarding rate control, the appar-ent oxygen corrosion rate, iaocr, is calcu-lated from Fick’s First Law according to Equation (2): iaocr = K nFC(O2)D(O2)/d (mpy) (2) where K = 5550 (conversion constant), n = number of equivalents per gram atomic weight, F = Faraday constant (96,500 A-s/equivalent), C(O2) = oxygen concentration (mg/L), D(O2) = oxygen diffusion coefficient (cm2/s), and d = dif-fusion thickness (cm).Using D(O2) = 2.24  10–5, C(O2) = 7.7, and d(ave) = 1.37, collecting terms and constants, Equation (2) simplifies to: iaocr = 5550 [(2.24  10–5] 7.7/1.36 ≈  0.7 mpy (0.0178 mmpy) (3) The icorr is substantially higher than iaocr by the ratio icorr/iaocr = 1.2/0.7 = 1.7, in-dicating that cathodic hydrogen dis-charge supports the corrosion process.4 Assuming the corrosion rate of com-pletely submerged wrought iron is 70% higher,1 the ratio increases to (1.2  0.7 + 1.2)/0.7 ≈ 2.86. This ratio (actual cor-rosion rate/corrosion rate controlled by oxygen consumption) is defined as the Weins Number, Wn.4 DiscussionBecause the port side (seaside) under-goes more intermittent splash than the starboard side, high corrosion rates are primarily responsible for the disappear-ance of hull metal on that side. Mechan-ical impact from wave-borne floating debris and abrasion are contributing fac-tors. The carryover appears similar to TAbLE 1Concretion sample location, properties, and corrosion rateSampleLocation (Metal)ρ(g/cm3)d(cm)Fewt%CECR(mpy) Comments06-002 Low stern(cast)2.24 0.98 57.59 0.8 CECR, exterior, broken, FeCO306-003 Midship(wrought)2.28 1.75 60.98 1.4 CECR, interior,  chiseled,  0.1 wt% Ca06-005 Aft, tower(wrought)2.30 0.80 48.01 0.5 CECR, interior, drill/saw,  4.3 wt% Ca06-006 Aft(cast)2.40 2.36 53.7 1.8 CECR, exterior, drill/saw,  0.3 wt% Ca06-007 Midship(wrought)— — — 1.8 Calipers, base metal06-018 Midship(wrought)3.01 0.95 48.94 0.8 CECR, interior, chisled16 wt% CaAverage 1.2 0.03 mmpyρ = concretion density (ASTM D792-00), d = thickness, Fe wt% = iron content, and CECR = concretion equivalent corrosion rate.Continued on page @@NACE International, Vol. 49, No. 10  October 2010 MATERIALS PERFORMANCE  53M AT E R I A L S  S E L E C T I O N  &  D E S I G Nflash corrosion wherein high oxygen leve l s  y ie ld  ferr ic  oxyhydroxide [FeO(OH)]. This corrosion product tends to dry out to form hematite and magnetic maghemite, the reddish-orange to red ferric oxides. Evidence of these oxides was observed in various forms depending upon the degree of dryness on the upper starboard hull. ConclusionsThe reported results are consistent with U.S. Navy corrosion data. The up-per limit to the deterioration rate of Sub Marine Explorer is unknown because of the extent of mechanical impact and abrasion. As discussed, the corrosion rate is variable depending upon pro-tection against corrosion that is pro-vided by concretion and the position of various parts of the hull in relation to tide flow pattern and flash corrosion from carryover. An average corrosion rate of 0.03 mmpy (1.2 mpy) is estimated for the ex-terior hull side. Assuming the same cor-rosion rate on the interior hull side, the average corrosion rate for both sides is nearly 0.061 mmpy (2.4 mpy). Based on data and observations on the upper star-board side, about one-half of the original thickness of the upper outer hull remains; structural integrity of this part of the hull structure is reaching critical stage. Cor-rosion rate control involves the interac-tion of oxygen consumption and hydro-gen discharge with the latter apparently being dominant. The authors recommend that cast and wrought-iron samples be obtained from the site for detailed metallographic ex-amination to: 1) confirm the assumption that the cast portion of Explorer is gray cast iron in terms of today’s classification, and 2) determine corrosion rate from the depth of the graphitized zone in the cast iron microstructure for comparison with results in Table 1.AcknowledgmentsThe analysis of Sub Marine Explorer was accomplished under permit by the Gov-ernment of Panama through the Instituto Nacional De Cultura in two separate field sessions in 2004 and 2006. We also ac-knowledge the financial support of the Council of American Maritime Museums (CAMM) and the Office of Ocean Explo-ration of the National Oceanic and At-mospheric Administration (NOAA). A subsequent field season completed docu-mentation of the craft in 2008 and was funded by Waitt Institute for Discovery.Archaeological survey team members in 2006 were James Delgado, Todd Cro-teau, Bert Ho, Michael McCarthy, Larry Murphy, Joshua Price, Don Johnson, and Karl Vandenhole. The authors also thank the NACE International Omaha Section for providing undergraduate scholarship funding for student laboratory assistance at the University of Nebraska-Lincoln, and the National Park Service Sub-merged Resources Center for its support. References1 I.D. McCleod, C.V. Ruppe, J.F. Barstad, eds., “In Situ Corrosion Measurements and Management of Shipwreck Sites,” Intl. Handbook of Underwater Archeology, pp. 697-714.2 M. Schumacher, ed., Seawater Corrosion Handbook (Park Ridge, NJ: Noyes Data Corp., 1979), pp. 13-14; 367-387. 3 ASTM D792-00, “Standard Test  Methods for Density and Specific  Gravity (Relative Density) of Plastics by Displacement” (West Conshohocken, PA: ASTM Standards, 2000).4 D.L. Johnson, D.J. Medlin, D.L. Conlin, L.E. Murphy, M.A. Russell, J.D. Carr, B.M. Wilson, “The Weins Number— Integrated Long-Term Corrosion Decay Trajectories of Iron-Based Alloy Ship-wrecks and Artifacts on the Sea Floor,” Research in Progress (RIP) Symposium Extended Abstracts, March 14-18, 2010, NACE CORROSION 2010, San  Antonio, Texas.Editor’s note: Sub Marine Explorer was recently featured in the Smithsonian Network’s “America’s Lost Submarine” and Speigel TV International’s “The Search for Explorer,” History U.K. and U.S. versions.DONALD L. JOhNSON is a volunteer metallurgical engineer/corrosion scientist at the Submerged Resources Center, National Park Service, 14709 W. Via Manana, Sun City West, AZ 85375, e-mail: johnsondonl@aol.com. he is also a professor emeritus at the University of Nebraska-Lincoln.  his recent experience includes corrosion of submerged resources; past experience includes teaching and research with publications in the areas of thermodynamics, hydrogen transport, and corrosion. he was lead engineer in the develop-ment of the Concretion Equivalent Corrosion Rate (CECR) concept pioneered during the USS Arizona Preservation Project. he was awarded the George B. hartzog National Park Service Volunteer of the Year Award in 2006. he is a 44-year member of NACE International.LARRY E. MURPhY is an archaeologist with Submerged Cultural Resource Consultants, PO Box 15065, Panama City, FL 32406, e-mail: lmurphy56@comcast.net. he previously was chief of the National Park Service Submerged Resource Center for 30 years, during which time he participated or led more than 100 underwater archeological research projects. he has an M.A. degree and received the Society for historical Archaeology Award of Merit.JAMES D. CARR is a professor emeritus of chemistry at the University of Nebraska-Lincoln, Department of Chemistry, Lincoln, NE 68588-0304, e-mail: jcarr1@unl.edu. Prior to retirement, he taught chemistry and directed graduate research in analytical chemistry. he has a Ph.D. in analytical chemistry from Purdue University and authored the textbook, Analytical Chemistry and Quantitative Analysis.BRENT WILSON is director of Research and Development at Amsted Rail, 1700 Walnut St.,  Granite City, IL 62040, e-mail: bwilson@amstedrail.com. he formerly was a professor at the University of Nebraska-Lincoln. he has a Ph.D. in chemical and materials engineering and is an eight-year member of NACE.JAMES P. DELGADO is president and chief operating officer at the Institute of Nautical Archaeology,  PO Drawer hG, College Station, TX 77841, e-mail: jdelgado@inadiscover.com. he is a maritime archaeologist who specializes in 19th century wrecks. he was the principal investigator for the Sub Marine Explorer archaeological missions and documentation of the site. View publication stats

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Corrosion of Civil War Era \u3ci\u3eSub Marine Explorer\u3c/i\u3e—Part 2

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