Hartt, William H.

The fatigue crack growth behavior of welded ABS DH32 steel spec~mens
cathodically polarized to -1.00 volt (Cu-CuS04) and subjected to bending
fatigue of nominal stress + 69 MN/m^2 (+- 10 ksi) in seawater was
investigated. Small non-propagating surface cracks with lengths up to
4.93 mm formed in the specimens. There were no s~gns of failure after
36 x 10^6 cycles when the experiment was terminated. Comparison of
this test with similar freely corroding and -0.85 volt (Cu-CuS04)
tests indicated that -1.00 volt (Cu-CuS04) cathodic protection is conducive
to arrestment of small fatigue cracks. This suggests that
modest cathodic over-protection of steel in seawater may beneficial.

The stress concentration which exists at the toe of an AWS class
'c' weld has been investigated by means of the finite element method.
This type of weld is representative of that specified to join tubular
members for offshore structures. Welded fatigue specimens were sectioned
and the weld profile described in terms of 1) the weld toe
radius (R), 2) the reinforcement angle (e) and 3) the filler metal
height QH). A statistical analysis was performed to determine the
correlation between R, e and H from which it was found that they are
independent of each other. By use of the finite element method, trends
were established for the effect each parameter has on the stress concentration
at the weld toe. Analysis of the effect of undercutting
was also perfonned and the results used to offer an explanation why
fatigue cracks propagate to failure in some cases and arrest in others.

Fatigue tests were performed at low stresss intensity ranges
upon precracxed HY-80 steel specimens in air and sea water,
at stress ratios of 0.1 and 0.8 with and without cathodic
protection. Crack growth. was found to be higher at R = 0.8
than R = 0.1. Also, the relative positioning of fatigue
crack growth rate data for freely corroding and cathodically
protected specimens reversed order cs R changed from
0.1 to 0.8. Threshold stress intensity ranges were found
to be higher in sea water than in air. A comparison of
the deltaKth for HY-80 was made with. that of other steels.

To investigate environmental conditions which influence the
type and formation of calcareous deposits, AISI 1010 steel
specimens were exposed in sea water at constant velocities
and controlled potentials. Chemistry and thickness of the
deposits which formed as a result of cathodic polarization
of steel in sea water were determined. These variables were
found to be a function of velocity and controlled potential
with film thickness decreasing as velocity increased for all
potentials considered. This behavior is interpreted in
terms of variations in pH at the metal surface. A mass
transfer analysis is presented which projects the pH at the
metal surface as a function of velocity and controlled potential.
Scanning electron micrographs are also included
to give information regarding the nature and type of deposits.
The experimental results are discussed within the
frame of accepted concepts of sea water cathodic protection.

Investigation of concrete-rebar interface conditions was
conducted for the purpose of identifying governing parameters
of the concrete cracking process in seawater. Experiments
included an examination of chloride ion penetration,
concrete pH as a function of metallic chloride concentration,
and pH distribution within reinforced, impressed current
specimens. Metal-concrete systems investigated included
iron, molybdenum, cadmium, zinc, aluminum, copper, and
nickel. Simplified models of the rebar corrosion process
are presented in conjunction with electrochemical analysis
of the projected interface chemistry. Results indicated
that chlorinities greater than seawater concentration may
occur as a result of potential gradients in reinforced concrete.
Laboratory simulations of interface chemistry indicated
that metallic and chloride ions may produce an acid
pH in concrete solutions when present in sufficiently high
concentrations. Heasurements of pH on the fracture face of
impressed current corrosion specimens indicated localized
acidic conditions can develop.

Double cantilever beam specimens of 7079-T651 aluminum were
subjected to low constant stress intensities in a sea water
environment to determine the stress corrosion cracking response.
In addition to a constant stress intensity some specimens
were subjected to controlled, constant potentials. Despite
the fact that all tests were in Region I of the crack growth
rate-stress intensity curve, where the former has been projected
to be very dependent upon the latter, a unique relationship
between stress intensity and crack growth rate was not
always indicated. Therefore, some variable other than stress
intensity is assumed to control crack growth, particularly
for the first several hundred hours of exposure. The observed
behavior is discussed in terms of accepted theories of stress
corrosion cracking in high strength aluminum alloys, including
electrochemical dissolution and hydrogen embrittlement.

High cycle fatigue tests were conducted using symmetrical
center cracked plate specimens in air, sea water and constant
potentials of -1.0V and -1.3V(SCE) at 20 Hz. (1200 cycles per
minute) to determine what effect these environments had on
crack growth rates, delta a/delta n (inches per cycle), of 5086-H34
aluminum when plotted against stress intensity range, delta K (Ksi root of in). Crack growth rates of 10^-8 inches per cycle
were obtained which indicated that apparent threshold stress
intensities, delta K th, exist which are the same for air, sea
water and a constant potential of -1.0V(SCE); however, under
a constant potential of -1.3V(SCE) the threshold shifts to a
significantly higher value. It was also observed that the
different enviroments had a pronounced effect on crack
growth rate only at low stress intensities. Several mechanisms
are considered to explain this behavior.

The objective of this investigati.on was to contribute to
the determination of a suitable metallic coating for steel
reinforcement in concrete exposed to a marine environment
and possibly to stray current. Concrete specimens embedded
with steel, aluminum, cadmium, copper, molybdenum, nickel,
and galvanized steel were exposed to sea water. Direct
current was impressed on these to accelerate corrosion of
the reinforcement so that cracking of the concrete could be
observed within the time limit of the investigation. Aluminum,
cadmium, copper, and nickel were found to have no beneficial
effect on concrete cracking as compared to steel. In
addition, no correlation between oxide-metal volume ratio
and concrete time-to-cracking could be observed. It was
found that corrosion of molybdenum has little or no tendency
to crack concrete and its corrosion potential and active
behavior approximates that of steel, indicating it may be
an excellent coating.

Reverse bend, constant deflection fatigue experiments
were conducted in natural sea water at 1850 cycles per minute
(cpm) to investigate the response of 1018 notched steel to
concurrent cyclic loading and cathodic polarization. For
comparison purposes S-log N curves were also determined in
air and high purity argon. These indicate that the endurance
limit established in sea water by cathodic polarization at
-0.85 volts, Saturated Calomel Electrode (SCE), is greater
than in either air or argon. Two mechanisms are considered as
responsible for this behavior.
1. Presence of carbonate and hydroxide deposits
coupled with increased alkalinity at non-propagating
crack tips.
2. Adsorbed OH- layer at non-propagating crack tips.

It is recognized that reinforced concrete structures are
often susceptible to stray current corrosion damage. Experiments
have been performed upon a reinforced concrete bridge
model in sea water, and these indicate that 0.01 percent of
stray direct current in this electrolyte may enter the structure.
Alternating current may also contribute to corrosion
of reinforcing steel, though experimental results reveal detection
to be more difficult than for direct current. Two
other sources of current in reinforcing steel are from cathodic
protection systems of utility attachments and from galvanic
coupling of the covered steel and bare steel. Mechanisms
by which the above types of current contribute to deterioration
of reinforced concrete structures are discussed.