Eng. &Tech.Journal, Vol.33,Part (A), No.

5, 2015

Effect of Prior Corrosion on Fatigue Life of Steel Alloy CK35

Under Constant Loading

Dr. Alalkawi H.J.M

Electromechanical Engineering Department, University of Technology/Baghdad
Dr. Abdul-Jabar H. Ali
Al-Khwarizmi Engineering College, University of Baghdad /Baghdad
Email:Dr.abduljabarha@yahoo.com
Saisaban A. Fahad
Engineering College, University of Al-Mustansiriya/Baghdad
Email:Saisaban.fahad@gmail.com

Received on:4/5/2014 & Accepted on:5/3/2015

ABSTRACT:
Corrosion is a term defined as oxidation of a metal, which is observed in many parts. A
structure is subjected to fatigue loading when a certain load is applied repeatedly. If this
structure which was subjected to prior corrosion, under fatigue load, it reaches
destruction far faster than that subjected in an inert environment. The aim of this work is
to study the effect of the prior corrosion on fatigue behavior of steel alloy CK35 under
constant amplitude stress. The NaCl solution used in this investigation is a 3.5wt% mass
mixture of sodium chloride (NaCl) salt and distilled water corresponding approximately
to the composition of sea water. The samples are immersed in the solutions for 80 days
on the solution that replaces every eight days to maintain the concentration of the
solution. The results observe that the fatigue life decreases in different percentage at
different constant stress amplitude loading. The highest decreasing life factor is in range
(9-29.3%) at amplitude stress in range (77- 62% of ) while the decreasing life factor is
in range (68.7-40.2%) at amplitude stress in range (46-31% of ). Also the experimental
results show that prior corrosion for 80 days reduce the fatigue limit of steel alloy CK35
for 13.229%.

Keywords: prior corrosion, fatigue life factor, S-N curve, steel alloy CK35

‫ ﺗﺤﺖ ﺣﻤﻞ ﺛﺎﺑﺖ‬CK35 ‫ﺗﺄﺛﯿﺮ اﻟﺘﺄﻛﻞ اﻟﻤﺴﺒﻖ ﻋﻠﻰ ﻋﻤﺮ اﻟﻜﻼل ﻟﺴﺒﯿﻜﺔ اﻟﺼﻠﺐ‬
:‫اﻟﺨﻼﺻﺔ‬
‫ ﯾﺨﻀﻊ اﻟﮭﯿﻜﻞ اﻟﻰ ﺣﻤﻞ اﻟﻜﻼل ﻓﯿﻤﺎ اذا‬.‫اﻟﺘﺂﻛﻞ ھﻮ ﻣﺼﻄﻠﺢ ﯾﻌﺮف ﺑﺄﻧﮫ أﻛﺴﺪة اﻟﻤﻌﺪن واﻟﺬي ﻟﻮﺣﻆ ﻓﻲ ﻋﺪة أﺟﺰاء‬
‫ إذا ﻛﺎن اﻟﮭﯿﻜﻞ اﻟﻤﺘﻌﺮض اﻟﻰ ﺗﺄﻛﻞ ﻣﺴﺒﻖ ﺗﺤﺖ ﺗﺄﺛﯿﺮ ﺣﻤﻞ اﻟﻜﻼل ﻓﺄﻧﮫ ﯾﺼﻞ ﻟﻠﻔﺸﻞ‬.‫أﺳﺘﺨﺪم ﺣﻤﻞ ﻣﻌﯿﻦ ﺑﺸﻜﻞ ﻣﺘﻜﺮر‬
‫ ان اﻟﮭﺪف ﻣﻦ اﻟﺒﺤﺚ ھﻮ دراﺳﺔ ﺗﺄﺛﯿﺮ اﻟﺘﺂﻛﻞ اﻟﻤﺴﺒﻖ ﻋﻠﻰ ﺳﻠﻮك اﻟﻜﻼل‬.‫أﺳﺮع ﺑﻜﺜﯿﺮ ﻋﻦ ذﻟﻚ اﻟﻤﺘﻌﺮض ﻓﻲ ﺑﯿﺌﺔ ﺟﺎﻓﺔ‬
‫ اﺳﺘﺨﺪم ﻓﻲ اﻟﺒﺤﺚ ﻣﺤﻠﻮل ﻛﻠﻮرﯾﺪ اﻟﺼﻮدﯾﻮم ﺑﻨﺴﺒﺔ ﺧﻠﻂ‬.‫ ﺗﺤﺖ ﺳﻌﺔ اﺟﮭﺎد ﺛﺎﺑﺖ‬CK35 ‫ﻟﻌﯿﻨﺎت ﻣﻦ ﺳﺒﯿﻜﺔ اﻟﺼﻠﺐ‬

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

‫ ﻣﻦ ﻛﻠﻮرﯾﺪ اﻟﺼﻮدﯾﻮم واﻟﻤﺎء اﻟﻤﻘﻄﺮ اﻟﻤﻜﺎﻓﺊ ﺗﻘﺮﯾﺒﺎ ﻟﻠﻤﯿﺎه اﻟﻤﺎﻟﺤﺔ ﺣﯿﺚ ﻏﻤﺮت اﻟﻌﯿﻨﺎت ﻓﻲ اﻟﻤﺤﻠﻮل‬٪3.5 ‫وزﻧﯿﺔ‬
‫ أظﮭﺮت اﻟﻨﺘﺎﺋﺞ أن‬.‫ ﯾﻮﻣﺎ ﻋﻠﻰ ان ﯾﺴﺘﺒﺪل اﻟﻤﺤﻠﻮل ﻛﻞ ﺛﻤﺎﻧﯿﺔ اﯾﺎم ﻟﻠﻤﺤﺎﻓﻈﺔ ﻋﻠﻰ ﺗﺮﻛﯿﺰ اﻟﻤﺤﻠﻮل‬80 ‫اﻟﻤﺴﺘﺨﺪم ﻟﻤﺪة‬
‫ ﺣﯿﺚ ﻛﺎن اﻋﻠﻰ اﻧﺨﻔﺎض ﻟﻤﻌﺎﻣﻞ‬.‫ﻋﻤﺮ اﻟﻜﻼل اﻧﺨﻔﺾ ﺑﻨﺴﺐ ﻣﺨﺘﻠﻔﺔ ﺑﺤﺴﺐ ﺳﻌﺔ اﻟﺘﺤﻤﯿﻞ اﻟﺜﺎﺑﺘﺔ اﻟﻤﺴﻠﻄﺔ ﻋﻠﻰ اﻟﻌﯿﻨﺎت‬
‫( ﻣﻦ اﺟﮭﺎد‬٪62 - 77) ‫( ﻣﻦ ﻋﻤﺮ اﻟﻜﻼل ﻓﻲ ﺑﯿﺌﺔ ﺟﺎﻓﺔ ﻋﻨﺪ ﻧﺴﺒﺔ ﺳﻌﺔ ﺗﺤﻤﯿﻞ‬٪29.3-9) ‫ﻋﻤﺮ اﻟﻜﻼل ﺗﺴﺎوي ﻣﺎ ﺑﯿﻦ‬
(٪40.2-68.7) ‫ ﻓﻲ ﺣﯿﻦ ﻛﺎن اﻧﺨﻔﺎض ﻣﻌﺎﻣﻞ ﻋﻤﺮ اﻟﻜﻼل ﺑﻨﺴﺒﺔ ﺗﺴﺎوي ﻣﺎ ﺑﯿﻦ‬،‫اﻟﺨﻀﻮع ﻟﻤﺎدة اﻟﻌﯿﻨﺎت اﻟﻤﺴﺘﺨﺪﻣﺔ‬
‫ ﯾﻮم‬80 ‫ ﻛﻤﺎ أظﮭﺮت اﻟﻨﺘﺎﺋﺞ اﻟﻌﻤﻠﯿﺔ أن اﻟﺘﺂﻛﻞ اﻟﻤﺴﺒﻖ ﻟﻤﺪة‬.‫( ﻣﻦ اﺟﮭﺎد اﻟﺨﻀﻮع‬٪31 - 46) ‫ﻋﻨﺪ ﻧﺴﺒﺔ ﺳﻌﺔ ﺗﺤﻤﯿﻞ‬
.% 13.229 ‫ ﺑﻨﺴﺒﺔ‬CK35 ‫ﻟﺪﯾﮫ ﻓﻌﺎﻟﯿﺔ ﻟﺘﻘﻠﯿﻞ ﺣﺪ اﻟﻜﻼل ﻟﺴﺒﯿﻜﺔ اﻟﺼﻠﺐ‬

INTRODUCTION:

M any kinds of structures used in marine environments, such as ships, off shore
platforms, drilling rigs, harbor works, and underwater pipelines, are made from
carbon and alloy steels. Even though such steels are susceptible to corrosion,
they are widely used because of their relatively low cost, ease of fabrication, availability,
and range of strength levels. Since steels are subjected to corrosive degradation in marine
environments, the loss in fatigue resistance due to corrosion must be taken into account in
engineering design, or protection from environmental attack must be employed [1]
Chih-Kuang Lin and I-Lon Lan (2004) [2] investigated the influence of environmental
factors, including pH, chloride ion, and pitting inhibitor, on the fatigue properties of AISI
347 stainless steel. Systematic fatigue tests, including both high-cycle fatigue (HCF, S-N
curves) and fatigue crack growth (FCG, da/dN-K curves), have been conducted in air and
several aqueous environments. Results showed the HCF strength is markedly reduced in
an acid solution and in a chloride-containing solution, as compared to the air value.
Moller et al.(2006) [3] studied the corrosion effect of a low carbon steel (SAE 1006)
in natural seawater and various synthetic seawaters. It was found that the steel corroded
nearly four times faster in a 3.5% solution than in natural sea water. The corrosion rate
after immersion in synthetic sea water is similar to the corrosion rate after immersion in
natural seawater
Marcelino and Herman (2010) [4] investigated the corrosion behavior on the reverse
bending fatigue strength of AISI 4130 steel used in components critical to the flight-
safety. The tests were performed on hot-rolled steel plate specimens, with load ratio R =-
1, constant amplitude, 30 Hz frequency and room temperature. It was observed that the
reverse bending fatigue strength of AISI 4130 steel decreases due to the corrosion.
Kang et al. (2011) [5] analyzed the fatigue crack propagation in high performance
steel, HSB800 in air and seawater environments using three-point single-edge notched
bending fatigue tests. A 3.5% sodium chloride solution was used as the seawater, and
several types of loading conditions, according to the stress ratio and loading frequency,
were applied. The results showed that the corrosion fatigue crack in seawater was more
rapidly propagated than was that in the air environment.
Jianhua Liu et al (2011) [6] studied the effect of pre-corrosion on fatigue behavior of
high strength steel 38CrMoAl with a fatigue test method using the accelerated pre-
corrosion specimen in the neutral salt spray environment. Moreover, the fatigue behavior
of the steel for different pre-corrosion time is investigated by the axis-direction tensile
fatigue test. The fatigue life distribution characteristics of the pre-corrosion specimens are
studied using the statistical probability methods, and the mathematical expectations and
the standard tolerances of the material fatigue lives after different pre-corrosion time are

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

obtained. It is found that the crack initiation of the high strength steel is accelerated by
the preferential corrosion at the local plastic deform areas.
Zhao et al (2012) [7] studied the corrosion fatigue experiments on X80 steel in 3.5%
NaCl solution on a researcher-developed cantilever-bending corrosion fatigue testing
machine. The morphology of corrosion pits and the change in the surrounding
microstructure are carefully examined using SEM and TEM. The results showed that the
corrosion fatigue crack initiation is due to not only the corrosion pits’ stress concentration
effects but also the adhering corrosion products; the action of corrosion products on
fatigue crack initiation is explained using physical models.
Mohamed et al (2012) [8] investigated the effects of corrosion of a high mechanical
strength martensitic stainless steel that is used in aeronautic applications. HCF tests
(between 105 and 107 cycles) are carried out in two environments: (i) in air and (ii) in an
aqueous solution NaCl at a loading frequency of 120 Hz. Surface crack initiation is
observed in air, whereas in solution, the crack initiated at corrosion defects. The decrease
in fatigue strength due to corrosion is observed to be 33% at 107 cycles compared to the
same test conditions in air.
S .A .Al-Taher et al (2014) [9] investigated the pitting corrosion behavior of 304 and
316 stainless steel alloys in 3.5% NaCl solution Experiments were performed using
potentiodynamic polarization and potentiostatic techniques at room temperature.. The
fatigue strength of the pitted samples was determined using plain-bending fatigue test
machine. The fracture surface was also investigated using scanning electron microscope
(SEM).The results show that SS 304 is more susceptible to pitting corrosion and has
lower fatigue strength than SS 316 for the unpitted alloys samples. For both alloys, the
single pitted samples shows that a deterioration percentage in fatigue is of about 15%
while the multi pitted samples shows a deterioration percentage in fatigue of about 33%
compared to the unpitted samples.
The aim of this work is to study the effect of prior corrosion on the fatigue life of
steel alloy CK35 under constant amplitude stress.
Experimental work:
The tests for the chemical composition and mechanical properties of the steel alloy
CK35 used in this work is carried out at the Specialized Institute for Engineering
Industries of the Ministry of industry using spectrum analyses. The experimental
chemical composition of steel alloy CK35 with the standard chemical composition is
presented in Table (1) while the experimental mechanical properties with the standard
values are listed in Table (2).

Table (1): Chemical composition of steel alloy CK35 in wt. %

Element C Si Mn Ni S p Cr Cu As

Standard 0.32 -0.4 0.17 - 0.5 -0.8 max max max max max max
Values 0.37 0.3 0.04 0.035 0.25 0.3 0.08
(ASM)
Experimental 0.307 0.26 0.65 0.023 0.018 0.028 0.13 0.015 0.0035
Values

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

Table (2): Mechanical properties of steel alloy CK35.

Mechanical Yield strength Ultimate Elastic modules Elongation
properties ( ) strength ( ) ( ) %
Standard 650 740 206 12
Values (ASM)
Experimental 660 690 200 10
Values

Fatigue Test Specimens Preparation:

24 specimens were prepared according to DIN 50113, 12 specimens (without
corrosion) for dry fatigue and 12 specimens for corrosion-fatigue tests. Figure (1) shows
the configuration of fatigue test specimen. The surface of the specimen is smoothed by
using silicon carbide papers for finishing.

Immersion:
The NaCl solution used in this investigation is a 3.5wt% sodium chloride (NaCl) salt
and distilled water corresponding approximately to the composition of sea water.
The immersion are performed in continuously aerated solutions using three specimens
per solution. The volume of the solution is 2000 ml. The specimens are immersed in the
solutions for 80 days. The water in the test cells is refreshed every 8 days to maintain the
concentration of the solution.

Fatigue Tests Procedure:

All fatigue tests were carried out in the laboratories of electromechanical engineering
department, University of Technology using PUNN rotary fatigue bending machine for
more information see reference [10]. The experiments are conducted at room temperature
and at stress ratio R=-1. The stress ratio represents the minimum stress to the maximum
stress in each cycle.

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

Results and Discussion:

Specimens without and with corrosion were tested at constant stress amplitude with (-
1) stress ratio at room temperature to find the effect of corrosion on the experimental
fatigue life and to evaluate the corrosion fatigue life factor .The experimental results are
given in Tables (3 and 4) The S-N curve was obtained from these results as shown in
figure (3). The equation of power law regression is given by [11]:

= ….. (1)

Where
( ) is the applied stress, and (a),(b) are the fitting parameters. The regression constants
representative of the fatigue trends, from the model, and the fatigue endurance limit at
107 cycles are given in Table (5).

Table (3): basic S-N fatigue results (dry fatigue) at room temperature (RT)
Specimen No. Amplitude stress ( ) (Cycles) Average
1,2,3 0.77 2750
4,5,6 0.62 11317
7,8,9 0.46 300500
10,11,12 0.31 3750000

Table (4): constant stress Fatigue-corrosion interaction results at (RT)

Specimen No. Amplitude stress ( ) (Cycles) Average

13,14,15 0.77 2500

16,17,18 0.62 8000
19,20,21 0.46 94000
22,23,24 0.31 2240000

Table (5) Fatigue parameters and fatigue strength for low carbon steel alloy.
Description a b Fatigue strength at Reduction in
107 cycles ( ) Fatigue strength%
without corrosion 1211.5 -0.116 186.7 -
with corrosion 1428 -0.135 162 13.229

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

500

400
Pa)

σ= 1211.5Nf-0.116Dry fatigue
Appliedstress(M

300

σ= 1428-0.135 Corrosion fatigue

200

0 1000000 2000000 3000000 4000000

Number of cycle to failure
100

Number of cycles to failure (Nf)

Figure (3): (S-N) curve of dry fatigue and corrosion fatigue for steel alloy CK35 at
room temperature.

Corrosion- fatigue life factor (CFLF %):

It can be observed that the effect of pre corrosion on fatigue behavior, by estimating
the decreasing corrosion- fatigue life factor (CFLF %) at different amplitude stress. Table
(6) shows Corrosion- fatigue life factor (CFLF %) at different amplitude stress by using
equation (2).[12]
N − N
% = × 100 ....( 2 )
fD fC
CFLF
N fD

Where
( ) and ( ) is the dry and the corrosion fatigue life respectively. The results show
the effect of pre-corrosion on fatigue life of specimens under low loads tests have more
reduction as under (0.46 and 0.31 of ) tests than high loads tests as under (0.77
and 0.62 of ) . This reduction in fatigue life because the pitting which caused by
pre-corrosion on the surface of the specimens accelerate the crack initiation of fatigue
cracks and it clearly shows when the test loads under low loading tests.

Table (6): The Corrosion-fatigue life factor (CFLF) at different amplitude stress.
Amplitude stress Average Average Corrosion- fatigue
(Dry-Fatigue ) corrosion -Fatigue life factor (CFLF)
0.77 2750 2500 9%
0.62 11317 8000 29.3%
0.46 300500 94000 68.7%
0.31 3750000 2240000 40.2%

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

Scanning Electron micrograph (SEM):

Scanning Electron micrograph (SEM) has also have been used in order to observe the
damage in the steel alloy CK35. The examined for dry fatigue and pre corrosion-fatigue
samples have been done at the Center of Nano technology and advanced materials at the
University of Technology. The surface morphology of pre-corroded specimens differed
substantially from surface morphology of dry specimens without corrosion condition.
Figure (3) show a typical micrograph of what is observed ductile fatigue fracture for steel
alloy CK35 under dry fatigue test, while Figure (4) show a typical micrograph of what is
observed fatigue ductile fracture with incidents of brittle cleavage facets which indicates
some brittleness brought by prior corrosion for examination of the specimen surfaces of
the pre corrosion-fatigue test for steel alloy CK35. This finding is agreement for
decreasing of fatigue life for prior corrosion specimens.

Figure (3) shows a typical ductile fatigue fracture micrograph for steel alloy CK35

Figure (4): show micrograph of fracture surface morphology of the specimen

surfaces of the pre corrosion-fatigue test for steel alloy CK35.

CONCLUSIONS
The results show effect of prior corrosion for steel alloy CK35 with NaCl solution
used in this investigation on fatigue life with as;

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 Eng. &Tech.Journal, Vol.33,Part (A), No.5, 2015 Effect of prior Corrosion on Fatigue Life of Steel
Alloy CK35 Under Constant Loading

1.Fatigue life of corroded specimens is greatly decreased especially at high stresses for 80
days submerging in 3.5% NaCl. The decreasing of Corrosion- fatigue life factor (CFLF)
is in range (9-29.3%) and (68.7-40.2%) at constant stress amplitude loading in range (62-
77% of ) and (31- 46% of ) respectively.
2.The reduction in fatigue strength is 13.229% by pre-corrosion for 80 days for material
used in this work.
3.Scanning Electron micrograph (SEM) results shows fatigue ductile fracture with
incidents of brittle cleavage facets which indicates some brittleness brought by prior
corrosion.

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