Hartt, William H.

In order to investigate the cracking tendency of
cathodically polarized SEACURE superferritic stainless
steel, a series of potentiostatically controlled tests were
conducted in flowing natural seawater. U-bend and tensile
specimens were used to determine the threshold potential and
limiting fracture stress.
The microstructure was examined using a metallurgical
microscope and a chemical analysis was conducted across the
grain boundary. These results were then correlated to
hydrogen assisted cracking, by performing SEM and EDAX
analyses on fractured U-bend and tensile samples. Finally a
model was developed which explained the predominantly
intergranular cracking in the static stress condition and
also accommodated the transgranular failure during dynamic
loading.

A series of experiments were performed on a single
tendon in a prestressed concrete slab for the purpose of
studying the susceptibility of the high strength steel to
brittle fracture as a result of cathodic protection. The
procedure first involved impressing an anodic current until
active potentials were recorded along the tendon length.
Subsequently, cathodic polarization of the midsection of
the tendon to -1.3 V(SCE) was effected. After a prescribed
time, the tendon was removed and sectioned into 15 ern
lengths. Each segment strand was then notched and strained
to failure, in three-point bending, in order to determine
any effect of cathodic protection on the mechanical properties
of the material. Differences in average failure loads
were compared to determine if a loss of load-bearing
capacity and brittle fracture occurred due to hydrogen
absorption.

A model for calcareous deposit induced fatigue crack
closure has been previously reported based upon the
criterion that the deposit thickness within the crack
equals or exceeds one-half the minimum crack opening
displacement. In the present paper an expanded and refined
model is proposed by considering (1) compaction of
calcareous deposits during the closure period of the stress
cycle and (2) the relationship between R ratio and
threshold stress intensity range. Compression tests upon
calcareous films grown on steel surfaces have been
performed, and these reveal the change in deposit thickness
as a function of stress. The implications of these models
to calcareous deposit induced fatigue crack closure are
discussed in terms of mechanical and electrochemical
parameters.

'J 'ht' c· f feet of seawater ,_tnd potential on the fatigue crack
initiation hehavior of several microalloyed and thermomPrhanically
processed steels has been investigated. Five
steels with yield strength in t~e range 500 to 750 MPa (73
to 108 ksi) were selected which included a range of processing
histories from conventional quenching and tempering to
precipitation hardening and direct quenching. These steels
were chosen because of their potential utility as materials
of construction for deep water compliant platform hull and
riser systems. Blunt notch compact tension specimens were
exposed to natural flowing seawater under constant load ampli
tnrlc. conditions at both freely corroding and cathodically
protected potentials. Cycles to crack initiation were
characterized by cyclic stress intensity range normalized
relative to the square root of notch radius. Test data are
presented and compared with data from other tests and steels
using appropriate stress analysis methods.

In the present research butt welded ABS DH-32 steel
specimens were fatigue tested under freely corroding and
cathodic protection conditions by a time-series simulated
wide-band spectra of a North Sea wave climate. Previous
research pertaining to the probabilistic features of wave
dynamics, wave force models, Markov-Chain sea state
evolution and time-series simulation of wide-band spectra
are addressed as a background review. The resulting freely
corroding data are discussed on the basis of the SN plot
which developed from the previous investigations at FAU.
Calculation of the damage of failed specimens by employing
the "Equivalent Narrow-Band Approach" resulted in excellent
agreement with the linear damage summation assumption.

Initiation of corrosion of reinforcing steel in concrete is
often caused by chlorides. Using a pressurized method for
the purpose of accelerating penetration of sea water into
concrete, the threshold chloride ion concentration for
corrosion of reinforcing steel in Type I portland cement
concrete has been studied in detail. The variables that
have been investigated include water-cement ratio and steel
surface preparation. When corrosion was detected by
electrochemical potential measurement, the test was
terminated; and chloride ion concentration was evaluated. No
correlation between threshold concentration and water-cement
ratio was found. With regard to surface condition, the pre-rusted
steel specimen showed a tendency to corrode at a
lesser chloride ion concentration than for the other preparation
techniques, which were sand blasting and
pretreatment in a saturated calcium hydroxide solution.
The results are presented and discussed within a perspective
of established concrete and corrosion technologies.

Electrochemical conditions within corrosion fatigue cracks
are thought to exert an important influence upon
propagation rate. For the case of cathodically polarized
steel in sea water, it has been observed that pH of
electrolyte adjacent to steel increased and calcareous
material formed on the steel surface. As a consequence of
the latter process the electrochemistry within
cathodically polarized cracks may be unique compared to
other electrolytes. The objective of this study has been
to measure the potential and pH within simulated fatigue
cracks of ABS DH 32 steel under cathodic protection. In
addition, the calcareous deposits which formed within the
simulated cracks were observed and analyzed. Based upon
these data, potential, pH profiles and calcareous deposit
thickness and composition were measured. This, in turn,
permitted conditions within the simulated crack to be
better understood.

The object of this work was to investigate the fatigue
performance of 25.4mm thick, butt welded ABS DH-32 steel
plate under spectrum loading, which simulated the
environmental conditions of the North Sea. Specimens were
tested in sea water under different levels of cathodic
protection. The results, including the previous freely
corroding data, are discussed with regard to the effect of
cyclic - frequency, loading history, weld profile and
cathodic protection. This study indicated that load
interaction effects are negligible in the present
experiments. However, frequency, weld geometry and
cathodic protection significantly effect fatigue life.

Reverse bend, uniform stress beam specimens of ABS DH-32
steel were subjected to fatigue cycles of randomly generated
amplitude in a sea water environment to determine the effect
of narrow band spectrum loading on cycles to failure. The
loading spectrum was the C/12/20 spectrum, developed at the
National Engineering Laboratory in Scotland to simulate the
severe conditions of the North Sea. Some misinformation
which has been published by NEL and other& regarding ~he
C/12/20 spectrum is addressed. A number ot means for
presenting random amplitude cycles to failure data in the
form of an SN plot are discussed. A means for taking into
account fatigue test cycling rate in failure predictions is
proposed, and the results of the freely corroding specimens
of this investigation conform to the proposed technique.
The random amplitude specimens exhibited reduced fatigue
lives compared with similar specimens of an earlier constant
amplitude investigation.

It is well recognized that the electrochemistry modifications
within cathodically protected fatigue crevices in
seawater has pronounced effect on crack growth rate. This
study focuses its attention on potential distribution within
simulated fatigue crevice as a function of electrolyte velocity,
fatigue frequency and cathodic protection potential.
Results of salt water electrolyte and seawater electrolyte
are compared to bring about the effect of calcareous deposits,
Finally, a mathematical model based on mass transfer
laws is developed to rationalise potential distribution
trends within fatigue crevice and are compared with the
observed behaviour.