LITERATURE REVIEW-4A

The cross - correlation of eddy current images for detection of fatigue crack propagation by MOORE and others (2001), directs that there is a need within the NDE industry to calculate the growth of damage such as fatigue cracking and corrosion over the life of a structure. Charpy impact testing was performed to determine impact test and properties as a function of temperature,in one case to verify the compliance with requirement and in other case,as part of a temper embitterment assessment. Fracture toughness crack tip opening displacement (CTOD) testing was performed to obtain the fracture toughness required in the assessment. In the remaining cases the small samples were used to obtained tensile testing specimens and to perform direct metallographic evaluation to determine the nature and extent of the degradation that the steel suffered during service.A total of twelve half size Charpy specimens were machine from the samples that were extracted. Since these samples were small to carryout the Charpy impact tests, extension taps were welded to the end of the test materials where the v-notch was machined. The absorbed energy and the transmission temperature depend on the size of the Charpy specimen and since half size specimen were used, the correlation by API RP 579 was used. For half size specimen for the absorbed energy conversion of full size was achieved by multiplying by two. The procedure is to determined the transmission temperature and then add 11oC
(12oF) to account for the temperature shift in using half size specimens. The impact results at -12oC and -1oC (10oF and 30oF) are shown in figure 2.

LITERATURE REVIEW- 4

In statistical approach to underground corrosion carbon steel pipeline by KOGAS (2001) in field corrosion studies were that performed on polyethylene (PE ) coated carbon steel transmission pipeline laid  underground in Korea, Chemical and biochemical analysis of the soil adjacent to coating defects were performed in both fields on laboratory. The factor controlling corrosion were considered by linear regression analysis,Principal components (PCA) and multiple regression analysis,
The corrosion site had strongly positive correlation with the chemical factors such as low pH and high level of chloride,and anaerobic nature soil that promoted the growth of surface reducing bacteria (SRB). Characterized by low level of soil resistively and reduction - oxidation potential and finally on pipe to soil potential.It was proved that the statistical described results of field and laboratory works satisfactory. Finally the precision equation for the maximum corrosion depth of carbon steel and soil environment is presented. It is important that the chemical,biochemical and cathodic protection (CP) effects should be considered together for the precise prediction of corrosion behaviour i.e the environments.

LITERATURE REVIEW -3

The presence of a network of ferrite particles in an austenitic matrix acts as a barrier to crack propagation, thus enhancing the resistance to stress corrosion of the material. This effects associated to the morphology and distribution of the phase rather than its contents welding in a direction parallel to the stress using reductively high heat input improved,the stress corrosion of the material even further. The HAZ ( heat affected Zone) of AISI 304 Steel was highly susceptible to stress corrosion in chloride solution. The presence of carbide precipitates in the austenite grain boundary deteriorated the corrosion resistance of the steel, as they promoted anodic dissolution and the development of the stress corrosion cracks .
A recent literature review by KOVACH: and others (1993) identified thirty seven publications involving MIC - micro biologically influence corrosion) and stainless steel focus on those which involved the high performance stainless and stainless steels in an effort to determined whether or not MIC poses a threat to the engineering application of these materials .It is easy to summarize the characteristics which are generally regarded as representative of MIC  and help in distinguishing MIC activity. These characteristics have been identified based on service experience and were first detailed by Tatnall (3) MIC which is characterized by:
        (a) The pressure of the distinctively shaped coloured deposits that are usually soft and easily                     deformed.
        (b) Large cavities under pinhole surface penetrations or open pitting,gouging, and stunningly.
        (c) Frequent association with welds.
        (d) Rapid rate of penetration.
However, it is important to know that these corrosion characteristics are not limited or attack by MIC.
It is generally accepted that opposite identification of MIC requires confirmation of the presence of aggressive microbial species in substantial quantities and the elimination of other possible corrosion mechanism.

LITERATURE REVIEW - 1

In addition the effect of sulphur on GTA weld penetration was quantitatively evaluated by considering two heat of 304 stainless steel containing 90 and 240 ppm sulphur. Sulphur, as expected increased depth with ratio by altering the surface tension gradient driven flow in the weld pool. In the study of the influence of welding on the stress corrosion resistance of AISI 304 steel ,Barbosa,and others (1997) also specified that Austenitic stainless steel to code pressure vessels in the petrochemical plants.The materials is highly susceptible to stress corrosion in chloride environment,which can endanger the weld and lead to rupture of the component.In this work the calculation of the influence of welding parameters on the stress corrosion resistance of AISI 304 Steel exposed to a magnesium chloride solution. AISI 304 steel were manually welded using three different coated electrodes (AWS E 304-16, E308L-16, 316L-16) and two heat inputs (5.0 and 9.0kj/cm).
The welded sample were analysed by tensile strength test optimal microscopy and corrosion test carried out according to ASTM G36-73 guidelines. The result show s that AWS E309-16 electrodes produced the best results due to the microstructure of the resulting weld metal.

LITERATURE REVIEW

 Series of research have been established to advance the correlation of materials selection and good performance of welded steel in construction, fabrication and welding industries including production company which could be obtained through proper experimentation and analysis.To established a good competitive edge and proper customer service,construction and  production companies must have  reliable techniques for variability of corrosion correlation data.Dependability of service is one of the best performance measures by which a company can distinguish it self.
equipment shall be kept reliable without defective components in other to remain productive and competitive.The weld pool development during GTA and laser beam welding of the type 304 stainless steel,part II experimental correlation,(Zachariah and others,1989), highlights in part one of the paper, the result of the heat flow and the fluid flow analysis were presented in part II, of the paper,predictions of the computational model are verified by comparing the numerically predicted and experimentally observed fusion zone size and shape, stationary gas tungsten are and laser beam wide were made type 304 stainless steel difference times to provide a variety of solidification condition as cooling rate and temperature gradient.Calculated temperature rolling rates are correlated with the experimentation observed fusion structure.

SCOPE OF THE STUDY

Qualification for specification is limited to ASTM, ASME, NACE, ANSI and API standard and codes, This implies that the presentation of the research outcome shall conform to international and  professional recommendations for effective correlation acceptance and approval codes.This implies that the research confirmation  will be affected to fabrication, welding and selection of material for engineering design. The research would be limited to the test analysis of welds and parent metal reaction on sulphur reducing bacterial (SRB),critical crevice temperature (CCT) and critical pitting temperature (CPT) and preferential weld corrosion. Correlation data for welded steel will enable adequate provision of standard systems of serviceability factors.

OBJECTIVE OF THE STUDY-2

The objective propels the analysis of potential microbiological influenced corrosion(MIC) in stainless steel as a function of the resistance of stainless steel to localized corrosion as evaluated by conventional test methods.It is reasonable to explore whether or not all stainless that are equally susceptible to MIC.This is because of the wide range of pitting and crevice corrosion resistance available in stainless steels. There are many test which evaluate the resistance localized corrosion but the ferric chloride (ASTM G-68) method is one of the most widely used.This test produces acidic oxidizing condition similar to those produced by iron and manganese oxidizing types of bacteria. and as there may be some relationship between ferric chloride test data and the metal oxidizing types of bacteria. Ferric chloride testing has been used to determine pitting temperature (CPT) and critical crevice temperature (CCT) for corrosion resistance stainless steel and nickel based alloys (TATNAIL,1981).Ranking the materials by this and similar test has been helpful in making successful material selections. The correlation between the ferric chloride and a stainless steel chemical composition has been demonstrated in fig.1.Representing critical temperature are summarized on a range of stainless steel table.1, types 304 and 316 have relatively low resistance as indicated by low critical temperature,904L has moderate resistance and 6% Mo stainless steels have very good resistance, A study of Kovach and Raymond suggests that positive correlation exist between ferric chloride test and service experience(KOVACH,1995).
(1) Microbial activity almost certainly has been present in many of this applications,therefore this correlation suggest that high performance stainless steel are significantly more resistance to MIC than the standard grades.

OBJECTIVE OF THE STUDY

The objective of the  is to determine proper method for selection of materials.The construction of engineering project,design and structural activities may fail due to corrosion effect and fatigue failures. The selection of materials and effect of corrosion is confirmed  through experimentation and results  carried out  by engineer.Microbiological influenced  corrosion (MIC) has become accepted as form of corrosion that occurs in stainless steels.Reports during the last fifteen years have cited numerous examples of MIC as the explanation of corrosion failures in this steels.The most highly alloyed grades have been specifically designed for high resistance to localized corrosion. The high performance stainless has a long service history of successfully handling seawater in power plant condensers and in offshore drilling platform (KOVACH,1995).
 

PREFERENTIAL CORROSION DATA -AIM

The aim of the project were twofold :To evaluate the effectiveness of a rapid test which could be used after fabrication. 2. To identify a weld which is susceptable and secondly to assess the capability of the scanning vibrating probe. 3.To provide information on the corrosion of welds. Tests were conducted on three welds produced by TWI UK, and designed to exhibit specific corrosion behaviour,and on a weld that had failed in service. The rapid test conducted was the galvanostatic test whilst long time immersion test were conducted to provide reference data.Measurement of the microhardness were made to establish the extent of linkage with the corrosion behaviour.

PREFERENTIAL WELD CORROSION -2

The scanning vibrating probe is relatively new and could provide information about corrosion activity on a small scale. However, its utilization to investigate the corrosion of welds has weld limited. Pistorius  used a number of techniques including the scanning vibrating electrode to investigate the role of sulphur in the localized corrosion of welds in high manganese carbon steels (Pistorious,2000).The authors suggested that hardened microstructures within the HAZ,such as bainite and martensite,do not form galvanic couples with surrounding microstructure but the formation of the networks of sulphide in the fusion line area is the most likely explanation for grooving parallel to the weld line in corrosive environments.

PREFERENTIAL WELD CORROSION-2

There is a range of methods for testing the corrosion of property of weld and their sensitivity to preferential weld corrosion and these were reviewed by Griffiths and others(1999).These tests were generally designed to be conducted in a laboratory on a sample weld to provide confidence in the welding procedures. However in practice a welder may deviate from the specified welding procedure and produce a poor quality weld. There is a need for non-destructive techniques good for identifying a weld that is susceptible to preferential weld corrosion after fabrication .
The scanning reference electrode technique has been used to assess the preferential corrosion of weldment for use in offshore applications when freely corroding and with applied potentials. The technique was found to be sensitive and reliable compared to immersion and potentiostatic anodic dissolution tests. The results of the test were used to develop two indices for assessing susceptibility to preferential corrosion,the first index(R1) was calculated from the hardness of the  HAZ parent plate and the highest hardness of martensite that the parent form.The second index (ChE) used the copper ,nickel and silicon content of weld metal and parent plate to assess their relative nobilities.

THE PREFERENTIAL CORROSION -1

However,Bradley(1962) concluded that the silicon content of the weld had no direct effect on corrosion of steel wedments in sea water but had low hydrogen electrodes which would give rise to enhance corrosion of the weld.In such trend hicks,(1973) suggested the use of low alloy weld steels so that the weld is always cathodic to the base plate.Nevertheless,corrosion of the HAZ could still pose some problems for some application and will be dependent on the rate of cooling and Mn level in the steel,which affect the grain size and  the extent of the martensite formation.The net effect is that the HAZ could be anodic to the parent plate.Recent work at the welding institute (TWI) UK has shown the importance of nickel content in the weld metal compared to the parent plate with higher nickel content in the weld being percieved to result in the weld acting as a net cathodic with respect to adjacent parent plate. Macroscopic anodes and cathodes will develop only if there is sufficiently conducting medium, when the conducting medium is low or the medium is in the form of a thin liquid layer,the potential drop in the solution will be shorten, the distance over which anodes and carthodes can effectively interact.In those circumstances,significant localized corrosion can occur in regions that are percieved to be net cathodic,as has been observed

PREFERENTIAL WELD CORROSION

The preferential weld corrosion has been a major problems historically and remain a significant concern for industry. The term preferential weld describes selective attack of the weld itself rather than parent plate plate but is sometimes used in a somewhat loose way to describe corrosion of other features of a weldment for example the heat affected zone (HAZ).The origin of preferential attack is usually combination of more active weld material (the weld will tend to be more susceptible o corrosion than the parent material if in isolation) together with galvanic coupling to the parent material, The influence of the coupling is accentuated by the poor area ratio, i.e a small anodic weld and large cathodic based material. The key features of preferential weld corrosion,have been reviewed by Turnbull(1982) elsewhere.
   Lundin (1967), drew the following conclusion from a detailed investigation. In the welding of C- Mn steels , the silicon are the most anodic and unkilled steels are the most anodic and the unklled steels with the silicon content are the most cathodic. Of the weld metals,the normal basic coated electrodes exhibit the anodic potential and the acid types the most cathodic,while the potentials
for the weld metal from rutile electrodes are placed in an intermediate position.

CORRELATION OF CORROSION DATA FOR WELDED STEEL -ABSTRACT

Three test methods,namely immersion tests, galvanostatic anodic dissolution tests and the scanning vibrating probe technique,have been evaluated as adequate methods for ascertaining the susceptibility of welds in carbon steels to preferential corrosion. The used of hardness measurement by profiling and mapping was also evaluated. Three welds were tested for which the production has been controlled to produce difference types of corrosion behaviour. In addition a weld that had failed in service was used to validate the finding of the assessment.In the immersion test lasting nine months,the tree controlled sampled welds exposed to ASTM D114 seawater behaved as expected,in terms of which region of the weld was most susceptible to corrosion.