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Qualitative Corrosion Study of Standard Welds in Ship Repair Practice

Rafael Caraballo
National Engineering School (student)
Facultad de Ingenieria
Montevideo Urugray

Susana Rivero
National Engineering School (Assistant Professor)
Facultad de Ingenieria
Montevideo Urugray

José C. Cassina
Laboratorio de Soldagem &tecnicas Conexas (Ph.D. candidate)
Federal University of Rio Grande do Sul (RFRGS)
Porta Alegre-Brasil
email: katana@alfa.adinet.com.uy

Dedicated to the memory of Cap. Fritz Kuhlemann, Marine Engineer.

A Lab-scale test is presented to determine corrosion behavior in standard welds used in ship repair practice in Uruguay. Interest is focused in the Weld-HAZ-Base metal system with emphasis in metallurgical and corrosion characterization.

Test coupons of SAE 1010 steel with two edge preparation techniques are used; either mechanical (shearing) or thermal (Oxyacetylene cutting). Welds are SMAW butt-weld type, with root pass made with AWS 6010 electrodes and subsequent passes with AWS 7018. Weld is finished by gouging root and finishing with AWS 7018.

Coupons were put in a test tank designed to simulate navigation effects, using a wire rig to keep coupons semisubmerged in natural, non sterile sea water. Air injection induces excess of air and wave motion, producing a splash zone in the interface air-water. A sample unwelded coupon was included as reference. Test duration was of 22 days. The electrolyte was renewed in 48 hr. periods.

Each coupon has three different zones; the atmospheric one, in contact only with humid, salt-saturated air; the splash zone, with alternative wetting and drying in oversaturated oxygen conditions; the submerged zone, unaffected by wave motion.

Analysis techniques include: visual and metallographic analysis; corrosion speed and penetration rates, Mössbauer spectrometry, SEM analysis.

Attack is characteristic for each zone; atmospheric corrosion is developed downwards and parallel to weld; splash zone corrosion produces a thin anodic zone extending to borders of coupon; submerged zone has a random pattern where thermal history of plate and weld may be determinant

After scale removal , anodic zones are characterized by a very smooth surface.

The correlation coefficients and dispersion coefficients obtained for sheared samples may indicate certain influence of low temperature energy transfer in plate polarization behavior.

Mössbauer analysis indicates that weld coupons show the presence of hydrated oxides of greater formation enthalpy than unweld sample. Percentile distribution of oxides in the submerged and non-submerged zones indicate a faster oxide evolution in the latter. A variation in distribution between gas-cut sample and sheared one may sustain the possibility of greater energy present in the latter.

SEM observation permits the recognition of three basic types of oxides, whose structure can be correlated to Mössbauer results.

Testing procedures adjust, without major deviation to ASTM standards G1-72 and G4-68.

(1) Rafael Caraballo. Marine Engineering student at National Engineering School (Facultad de Ingeniería, Montevideo-Uruguay).

(2) Susana Rivero. Chemical Eng., Assistant Prof. Grade 3, National Engineering School (Facultad de Ingeniería, Montevideo-Uruguay), Head of Corrosion Lab at Chemical Engineering Department

(3) José C. Cassina. Mechanical Eng., MSc., Ph. D. candidate at the "Laboratório de Soldagem & Técnicas Conexas" (LS&TC) of the Federal University of Rio Grande do Sul (UFRGS) in Porto Alegre-Brasil, Assistant Prof. Grade 3, National Engineering School (Facultad de Ingeniería, Montevideo-Uruguay).

1. Technical revision.

Corrosion of ship hulls and structures and its impact on economic operation is a major concern of involved parties, in relation to structure resistance, propulsion resistance, fuel consumption and environmental, human and cargo safety.

Corrosion has been regarded as an economic factor comprising mainly maintenance aspects. However, the foremost classification societies are revising their position in this aspect in the light of recent catastrophic incidents and groundings.

The widespread use of high-tensile steel to develop lighter structures for the last 15 years, combined with a decrease in maintenance budgets to keep operative costs low has given rise to the need of raising survey standards for ships to keep class. Not only standards are more stringent, but better inspector training and new policies for free circulation of survey results are under way.

High tensile steel corrodes at a similar rate as common steel, which results in faster degradation of critical structures, since cross-sections of members and plate thickness is reduced. If localized attack is also considered, accident potential increases in unpredictable progression.

Present technology permits real time monitoring of hull fatigue and buckling, hull frame behavior, contaminant emissions, bilge and tank spills as well as electrochemical potential of hull. This also contributes to development of more stringent standards from involved parties and authorities such as Classification Societies, International Maritime Organization (IMO), Insurance Companies, National Coast Guards, etc.

Corrosion stands, then, as an operative and economic risk to successful ship operation to the builder. From this viewpoint, the development of better construction and repair methods regarding corrosion prevention and arrest is of capital importance.

This work aspires to contribute to the development of cost effective methods of corrosion prevention and testing.

1.1. Ship repair, welding and corrosion.

Basically, four factors are influential in corrosion processes in welds:[4]

Different composition and electrochemical potential in the Weld-HAZ-Base metal system.

Different metallurgical structures present in this system.

Joint geometry.

Mechanical stresses present.

Differences in composition and potential result in the formation of galvanic micro couples in the plate, depending on adequate selection of materials and procedures, that will ensure the cathodic nature of weld in reference to plate [5, 8].

However, HAZ has been found to be anodic in relation to the Weld-HAZ-Base metal system [8]. In the specific case of a ship, the effects of energy transmission by vibration, rolling, heeling, hogging, sagging and slamming should also be considered[7].

Testing should be done regarding everyday practice, i.e. on welds executed in actual working conditions, including execution defects and weld imperfections. This policy is indispensable to obtain valid results from tests carried out to select materials or procedures. [3, 6, 8]

Tests for evaluation of ship construction practices using tanks with aerated sea water for flow simulation, show that attack rates achieves a constant value dependent on diffusion speed in the oxygen reduction reaction. Good correlation with actual service conditions is observed.[8]

1.1.1. Welding Practice.

The standard process in ship repair welding in Uruguay is shielded metal arc welding - SMAW. To determine the most usual procedure, we asked experient technicians and engineers about it. Answers unanimously show the use of AWS 7018 - basic SMAW electrode, with iron powder addition - for welds in all positions. Border preparation may be either by mechanical means (shearing) if insert is of regular shape, oxyacetylene cutting, if insert is of special shape or a combination of both, if needed.

Joint profile is usually of U, V or X geometry, depending of plate thickness. This shape is attained by carbon electrode cutting, oxyacetylene cutting or machining. A variation is the use of AWS 6010 - cellulosic electrode - in the root pass, to achieve better penetration, subsequent passes of AWS 7018, with gouging of the root from the other side and finishing with AWS 7018. The first procedure is more adequate in hull framing and where access from both sides is inapplicable. The second procedure corresponds to hull welds, and will be used in the test.

Weld profile is as shown:

1.1.2. Influence of processes in metallurgic structure.

1.1.2.1. Cold working.

Empirical results indicate that energy used in cold working is degraded as 90% heat; the remaining 10% is absorbed into the crystalline structure, resulting in an increase of internal energy.[10, 12] This increase varies according to process and reaches a saturation value inversely proportional to temperature of process. In other words, with lower temperatures, greater saturation values are possible. This energy increase is responsible for generation and subsequent interaction of dislocations during deformation of metal. [9]

1.1.2.2. Oxyacetylene Cutting.

In the oxyacetylene cutting process, we should note that border preparation in this case constitutes a thermal treatment of the plate in uncontrolled conditions, with supply of a turbulent oxygen flow.

Some authors[11] state that their experience makes them suppose that gas cut surfaces are of equal or superior quality regarding corrosion resistance to cold-worked ones, since no violent structure deformation occurs during the former. Mechanical tests of fatigue resistance and Charpy notch impact resistance are also reputed as superior in case of gas cutting.

2. Testing Procedure.

2.1. Test modeling.

Modeling in a test such as the present one will result in compromise and approximation. The changing conditions of sea, wind, tides, currents, variations in temperature, wave period and patterns, ionic and biological concentrations cannot be duplicated in the lab. This test is supposed to be a reasonable approximation to the natural phenomenon.

Corrosion, in our case, occurs in structures too big to be lab-tested at actual scale. Modeling will result in border effects, changes in the statistical distribution of corrosion processes, flow scale problems and variations in electrical current distribution.[13]

Flow modeling is increasingly difficult, and keeping similitude between model and prototype cannot be done correctly at the same time. For example, in the case of cathodic protection of a given metal in sea water, the diffusion of oxygen through the boundary layer controls the reaction rate.[3] If we assume that the boundary layer is the same in model and prototype, current density should be the same in both. However, the boundary layer is difficult to identify, varying in different points, while scale formation cannot be appropriately controlled to keep similitude, resulting in variations that cannot be accurately accounted for.

These difficulties may well be impossible to overcome. Parameter selection must resort to common sense in establishing when the experimental conditions represent a good approximation.

2.2. Preliminary Manipulation .

There has been no extensive work in Uruguay regarding weld corrosion tests; with this in mind, this test has been planned to keep test conditions under control, to facilitate duplication or further investigation. All procedures were chosen for simplicity in manipulation and data recording.

For test purposes, coupons were prepared from SAE 1010 commercial steel plate with the following characteristics:

Coupons were approximately 5 mm thick. These plates can be easily manipulated to prepare metallography samples. Specimens were prepared in two groups:

The five lighter plates were sheared to size (200 mm by 100 mm) and then sheared again to obtain ten plates of 200 mm by 50 mm. Borders were cleaned with an iron bristle brush.

The five heavier plates were sheared to size (200 mm by 100 mm) and were kept until the moment of welding, when these were cut to 200 mm by 50 mm dimensions, to be immediately welded after cooling.

In the first group, we have typical cold work in border preparation. In the second, oxyacetylene border preparation is present.

All welds were butt-weld type, since for the involved thickness, no border preparation is mandatory, and would superimpose to the sheared or gas cut surface. Once weld, specimens were cut to final size (150 mm by 100 mm), to avoid transitory effects of arc ignition and interruption in weld; then were cleaned by air blasting with steel particles, to commercial finish, degreased and subsequently put in a desiccation container.

Coupons were fixed by electrically insulated wires to the walls of a cylindrical container of 500 mm diameter.

An electric stirrer was used to transmit a peripheral speed of about 5 knots. While this is not a specific speed, an increase would result in oxide removal, which would affect ionic concentration and diffusion mechanisms.

Specimens were put in vertical position, semi-submerged in electrolyte. in such a way that 50 mm of their length were completely submerged, 50 mm irregularly wetted by surface waves in water, and the remaining 50 mm only exposed to the spray coming from electrolyte surface.

To simulate aeration effects from heeling and wake formation, an air injector was fixed with neoprene glue to tank bottom, through which compressed air at low pressure was forced into tank. Air flow was regulated with a turning valve. The necessity to keep a low incoming air flow to maintain wave pattern stable, resulted in less aeration than what may have been appropriate to keep an uniform concentration near specimens. Diffusion and mixing mechanisms were more influential than direct aeration in oxygen transport to specimen surface.

Wave pattern was set by means of stirrer's R.P.M. and air flow from injector. Wave height was never superior to 15 mm, peak to peak

Samples of each border type were used to prepare inclusions for metallographic observation.

2.3. Development of test.

With the simulation tank running for the described condition, specimens were set in their positions for the duration of the test - 22 days-; sea water was changed every two days, to avoid increases in concentration of metallic ions that may affect corrosion reactions.

The relation electrolyte volume/specimen area was approximately 10 cm3 for each cm2 of area. A water sample for pH characterization was taken for the first fifteen days, since no important variations were registered, no further samples were taken. Sea water was collected in three locations: Playa Mansa (Punta del Este, dept. of Maldonado, Uruguay), Punta Colorada (Piriápolis, dept. of Maldonado, Uruguay) and Puerto Nuevo (La Paloma, dept. of Rocha, Uruguay).

3. Analysis and results discussion.

Being this a qualitative study, we recurred to three techniques for interpretation of results:

Corrosion rate, as resulting from mass loss (ASTM G1-72).

Mössbauer spectrometry.

Optical and SEM microscopy.

Attack is characteristic for each zone; atmospheric corrosion is developed downwards and parallel to weld; splash zone corrosion produces a thin anodic zone extending to borders of coupon; submerged zone has a random pattern where thermal history of plate and weld may be determinant

Mass loss rates were done for four samples of each group. For comparison effects, both total area and anodic net area were used for calculation. Characterization and measurement of anodic area was possible using photographs with a superimposed grille, with which a manual count of squares corresponding to anodic area by row was made. Total anodic area is the result of the sum of all computed rows.

Mössbauer effect analysis was done for the unweld specimen and one sample of each group.

Microscopical observation in both cases, was used for the characterization of particular differences in metallurgic structure and oxide morphology.

3.1. Corrosion Rates.

Calculations and procedures were carried out in accordance to standard ASTM G1 - 72 (ISO 8407 - 91 ANSI G80 1-1973) "Standard Practice for preparing, cleaning and evaluating corrosion test specimens", with deviation in the following:

1. - Specimen borders were not machined, with exception of mechanical brushing

3.3 - The necessary equipment to determine mass values to the fifth significant digit was unavailable.

3.1.1. Penetration Rate Values.

Penetration Rate considering total exposed area

Gas Cut Group	Rate (mm / year)	Sheared Group	Rate (mm / year)
C1	0.714	M1	1.080
C2	0.647	M2	0.732
C3	0.742	M3	0.420
C4	0.618	M5	0.576

Unwelded Sample: 0.764 mm / year

Gas Cut Group	Rate (mm / year)	Sheared Group	Rate (mm / year)
C1	1.501	M1	2.246
C2	1.301	M2	1.582
C3	1.750	M3	1.170
C4	1.413	M5	1.745

Unwelded Sample: 1.471 mm / year

The consideration of anodic area for calculation of attack rate gives a more accurate view of metal degradation. Values are duplicated and even tripled. This arises from better evaluation of localized attack in calculation. If used for methodical evaluation of structure damage, this method can be applied to failure prediction, specially if high tensile steel is used.

3.3. Scanning Electron Beam Microscope Analysis.

Pure goetite next to cathodic base metal

2500 X

Attack Morphology

5000 X

Lepidochrocite in goetite matrix Pure Lepidochrocite

1250 X 2500 X

3.4. Mössbauer effect analysis.

Theoretical reactions for corrosion indicates precipitation of Fe2O3. Nonetheless, the coexistence of an aqueous phase along with differential aeration implies the evolution of the following oxides.[18]

Oxide Type	Name	Formation Enthalpy (cal)
Fe(OH)3	Ferric Hydroxide	162.930
g FeOOH	Lepidochrocite	169.310
g Fe2O3	Maghemite	169.466
a FeOOH	Goetite (Fresh)	172.910
a Fe2O3	Hematite	177.400
a FeOOH	Goetite (Aged)	180.510

Each oxide type requires , for its formation, to give to the surrounding medium increasing amounts of energy. The percentage of oxides present in a given sample will indicate how much energy it has effectively surrendered. At least, it will be a way to estimate the evolution of the system to lower energy states. This can be visualized through the curves of potential / pH that follow, where we observe that although corrosion in our case will not stop after total evolution of oxides into stable forms, the whole system evolves to lower energy states.

	Unweld Sample	Sheared Coupon nº 4	Gas Cut Coupon nº 5
Non Submerged Zone	Lepidochrocite 54% Maghemite 7 % Goetite 39 %	Lepidochrocite 40% Maghemite 19 % Goetite 41%	Lepidochrocite 41% Maghemite 24 % Goetite 35 %
Submerged Zone	Lepidochrocite100%	Lepidochrocite 78% Maghemite 22%	Lepidochrocite 79% Maghemite 21 %

The presence of welds increases the quantities of the hydrate oxides maghemite and goetite, indicating a faster formation of more stable oxides. Regarding the formation enthalpy values and the percentage of oxide species, we can conclude that the weld-HAZ-base metal system has, previous to oxide evolution, increased its internal energy from successive processes and transformations.

In the non submerged zone of all specimens, the presence of goetite indicates a faster evolution than in the submerged zone. In the latter, only maghemite and lepidochrocite occur. Besides, maghemite only occurs in weld samples, which reinforces the notion of energy build up due to welding

The gas cut samples show less percentage of more stable oxides than sheared ones, indicating that the latter have more energy build-up, resulting from cold working processes. In the light of the small number of tested samples, this is not conclusive.

4. Conclusions.

The use of anodic area values gives a more realistic approach to penetration rate calculations, since localized attack effects are considered.

The aspect of anodic zones after scale removal can be used as a guide for optical quantification of its extension.

The presence of welds and their associated preparation processes produce an increase of the internal energy of the weld-HAZ-base metal system.

For sheared samples, the dispersion of penetration rates values, together with Mössbauer spectrometry values indicate a greater energy build-up for this group.

The precedent conclusion may sustain the suitability of oxyacetylene cutting for border preparation for better corrosion resistance.

Different oxide types can be identified in spite of morphologic variations. The origin of such variations cannot be determined and should be object of more specific tests.

Transition of one oxide species to another can be identified and characterized.

The flow model and test tank used were successful in generating different attack pattern, evolution and oxide morphology in a simple, cost-effective way

Observed attack in specimen is specific for each modeled zone, independent of sample groups. Variations occur only in the evolution of hydrate oxides present in each coupon

5. Acknowledgments.

To Dr. Chem. Eng. Ada Cabezas Soto, Chemical Investigations Centre, Ministry of Basic Industry, Cuba for her invaluable support and advice.

To Bach. L. Padrón, for his assistance in multiple operative and organization aspects of this work.

To Eng. Filippini for his collaboration in chemical analysis of electrolyte.

To Engs. D. Scuoteguazza, L. Della Mea and Mr. J. L. Vila for their contributions to this work.

To Engs. Quagliatta and Yelpo for the execution of Mössbauer spectrometry.

To Mar. Eng. A. Fernández for his accurate criticism and invaluable assistance in the execution of welds in ANCAP Plant - Fuel Division extensive to personnel directly involved in actual welding of samples.

To Mr. Carlos Di Perna, for his information.

To MAK S.A. for the facilitation of its premises for execution of shearing and blasting of coupons, extensive to personnel involved in the execution of these tasks.

6. Bibliography.

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3. -F.A.Champion; "Ensayos De Corrosión"; Ediciones Urmo
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