Corrosion Behavior of 7075-T651 Aluminum Alloy under

Different Environments

Beatriz Almeida Ferreirabeatrizalmeidaferreira@tecnico.ulisboa.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

November 2017

Abstract

Aluminum alloys are widely used in the aeronautical industry especially in the main airframe. Thesealloys are susceptible to corrosion, accompanied by embrittlement, that, if left untreated, can make anaircraft unairworthy. With this in mind, the corrosion behavior of bare and anodized 7075-T651 alu-minum alloy under a wet/dry cycle salt spray test in terms of loss of mechanical properties and fatiguelife was studied. Between the wet and the dry period, three different washing methods were considered:nitric acid, freshwater, and no wash. The effects of corrosion on the tensile properties revealed a generaldecay with increasing exposure time for ultimate tensile strength and yield stress. The Young’s mod-ulus did not have a significant variation and the ductility measures decreased exponentially generally.The fatigue life was reduced drastically by the anodization and tended to decrease exponentially withcorrosion as a result of premature crack initiation caused by pitting where pits are considered stressconcentrations. From the surface analysis, the primary corrosion mechanism detected was pitting. Thepits started to appear in random places all over the exposed surface and they coalesced, preferablyin the grain direction, with neighboring pits creating bigger and deeper pits. A practical applicationwas developed using the experimental results to characterize the corroded material and generate thenecessary material properties in the FEM solver standard language. These properties can be used forany simulation in any structure made out of the 7075-T651 aluminum alloy, within the test domain.Keywords: Aluminum Alloy 7075-T651, Salt Spray Test, Corrosion, Pitting, 7075-T651 TensileProperties, Fatigue Life

1. IntroductionAluminum alloys have been the main airframe ma-terials since they started replacing wood in the late1920’s [1]. The 7075 aluminum alloy is normally ap-plied in fuselage stringers and frames, upper wingstringers, floor beams, and seat rails and is sub-jected to corrosion. The corrosive action, whichcauses embrittlement, begins on the metal surfaceand moisture in the air is often sufficient to startcorrosion. When an airplane structure, constructedof several metals, is exposed to corrosive environ-ments such as exhaust gases, moisture, waste water,and spillage, necessary factors for corrosive actionare present and some areas of the airplane are ex-posed to more corrosive contaminants than others[2]. Left untreated, corrosion can make an aircraftunairworthy in just a few years.

With this in mind, this master thesis was cre-ated, to generate a general corrosion model of thismaterial using an autonomous underwater vehicle(AUV) that CEiiA is developing that has all hismain structure made by anodized 7075-T651 alu-minum alloy case study as a reference. This AUV

can represent a worst case scenario for the aeronau-tical industry.

The corrosion behaviour of bare and anodized7075-T651 aluminum alloy was studied through awet/dry cycle salt spray test, with different wash-ing methods. Then a quantification of corrosionwas made based on loss of mechanical propertiesand fatigue life.

To have knowledge of the corrosion behavior ofany material is important so that adequate mate-rials are chosen for different situations. Knowingtheir operational time, maintenance and restorationprocesses can be scheduled and applied correctlywhich will help to prevent failures.

As for scientific value, the tests presented in thisdissertation were never done before (at least pub-lished, based on the author’s research) and it canencourage other scientists to invest more in the con-ditioning variation test cycles instead of continuousexposure. In reality, there are many variations ofmany parameters and the test has to be as accu-rate as possible.

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2. Background2.1. Topic Overview

In 1996, Chen et al., [3], identified two types of con-stituent particles in the investigation of the role ofmicroconstituents in pitting corrosion in AluminumAlloy 2024-T3. Each type played a different role ininducing pitting corrosion, cathodic type promotesthe dissolution of the matrix around it and anodictype dissolve themselves. Frankel, in 1998 [4], stud-ied the factors that affect pitting corrosion and ex-plained its phenomena. The relation between pit-ting and fatigue did not take long to realize, andin 1996 Chen, Wan et al., [5], observed that fatiguecracks typically nucleated from one or two of thelarger pits, and the size of the pit at which the fa-tigue crack nucleates is a function of stress level andload frequency. Pits were identified as crack originsin all corroded specimens also in Jones and Hoepp-ner, 2004 [6], study. They realized that it was notalways the largest pit that formed/nucleated fatiguecracks. The combined effects of pit depth, pit sur-face area, and proximity to other pits were found toreduce fatigue life considerably.

To analyze environmental factors that affect fa-tigue life Chlistovsky et al., 2007 [7] subjected spec-imens to fatigue testing while they were fully im-mersed in an aerated and recirculated 3.5 wt% NaClsimulated seawater solution. A damage analysisshowed that the presence of the corrosive environ-ment accelerated the damage accumulation rate toa greater extent than that observed in air, partic-ularly at low stress ranges. Cavanaugh et al., 2010[8], included in his study variables like temperature,pH, [Cl−], exposure time, and orientation. Effectsof these variables in affecting maximum pit depthand maximum pit diameter for high-strength alu-minum alloy 7075-T651 were studied and reportedand it was found that exposure time and pH, fol-lowed by temperature, were the most significantvariables impacting pit depth.

The effect of anodic oxidation on fatigue perfor-mance of 7075-T6 alloy for pre-corroded and non-corroded specimens has been investigated by Cirikand Genel, 2008 [9], and the results indicate thatthe anodization has a tendency to decrease the fa-tigue performance. In 2013, Hemmouche et al. [10],studied the effects of some heat treatments and an-odizing processes on fatigue life of aluminum alloy2017. The result of fatigue tests showed a decreasein fatigue life of anodized specimens as comparedto untreated ones that could mainly be attributedto the brittle nature of oxide layer and to the het-erogeneous microstructure of the film.

Later, in 2015 [11], Dejun and Jinchun did a saltspray corrosion test on 7475 aluminum alloy bareand anodized. The results showed that the cor-rosion in the original sample surface is severe after

salt spray corrosion, while the anodic oxide film wasonly slightly corroded, owing it to Al2O3 which pre-vents Cl− to contact the base metal effectively.

2.2. Aluminum’s Passivation

In the pH range of about 4 to 8.5, aluminum is pas-sive (protected by its oxide film). Out of the passi-vation limits, aluminum generally corrodes in aque-ous solution due to the dissolution of its oxides inacids or bases, however, sometimes, even though itis outside of the passivation zone, the film is insolu-ble or the oxidizing nature of the solution maintainsthe oxide layer [12].

At low nonoxidizing potential the metal itself, Al,is stable, this is called the immunity zone. On thepassivity zone, the aluminum oxide, Al2O3, is stablewhich origins the protective layer. At low pH andhigh pH the aluminum ion, Al3+, and the aluminateion, AlO−

2 , respectively, are stable, which means themetal is subjected to corrosion on these areas [13].

2.3. Pitting Corrosion

Pitting is the most common form of corrosion foundon aluminum and it is usually manifested by therandom formation of pits [12]. Depending on theircomposition and environmental conditions the dif-ferent alloys are affected differently. In general, thepurer the alloy the higher pitting resistance it hasbecause pitting occurs as a galvanic reaction be-tween different elements on the alloy.

It is often the most damaging form of corrosiondue to the ability of perforation through the depthof the metal which results in loss of strength prop-erties through means of increased stress concentra-tions at these locations.

When the oxide layer breaks down the exposedmetal gives up electrons easily and the reaction ini-tiates tiny pits with localized chemistry supportinga rapid attack. This attack will cause the pit prop-agation (Figure 1).

Figure 1: Mechanism of Pitting Corrosion of Alu-minum (Source: [14])

Outside the cavity the reduction of water (23O2 +

3H2O + 6e− → 6OH−) and of H+ (6H+ + 6e− →3H2) occurrs, which will locally lead to an alkalinepH [14].

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At the pit’s bottom aluminum oxidation will oc-cur, this creates an electrical field that shifts theCl− ions towards the pit bottom forming aluminumchlorides. The aluminum chlorides then suffer hy-drolysis forming aluminum hydroxides (Al(OH)3)plus excess of hydrogen and chloride ions (H+,Cl−). This will lead to the acidification of the pit’sbottom and the media will become very aggressivemaking the pit to autopropagate [14]. Because ofthis, pitting is considered to be an autocatalyticprocess which means that once it has initiated, italters the local conditions to promote further pitgrowth.Al(OH)3 will precipitate and pushed to the open-

ing of the pit where it forms a deposit of white spots[14].

2.4. AnodizationSince the aluminum has a natural protective layer,one of the methods to slow down the corrosion pro-cess is thickening that layer. This is the principleof anodization. The layer is grown by passing a di-rect current through an electrolytic solution, withthe aluminum object serving as the anode, creatingmore aluminum oxide and, therefore, thickening thelayer. This increases abrasion and corrosion resis-tance and also provides better adhesion for paintsand primers [15].

3. Experimental Procedure3.1. Specimen DescriptionThe material used in this experiment was the Alu-minum Alloy 7075-T651. The specimens were cho-sen in accordance with the ASTM B557M, Stan-dard Test Method for Tension Wrought and CastAluminum and Magnesium Alloy Products (Metric)[16], as the Rectangular Tension Test Specimens,Standard Sheet-Type 12.5mm Wide.

The specimens were machined by an AWEA 3axis CNC BM 1020F from 600× 400mm aluminumsheets with 3 mm in thickness. To help with vibra-tional issues a 600×400mm vacuum table was usedto hold down the sheet during machining.

After machining, half of the specimens were an-odized according to AMS2469 which is a standardfor hard anodic coating on aluminum and aluminumalloys.

The test has a total duration of 20 cycles. Fiveexposure times were considered (1 cycle, 3 cycles, 9cycles, 15 cycles and 20 cycles) to obtain a more ac-curate tendency line of the loss of mechanical prop-erties and fatigue life. For each time interval, thirty-six specimens were manufactured, eighteen for fa-tigue testing and eighteen for tensile testing. Thespecimens were further divided into anodized andnot anodized and on the three washing methods.To get representative average values of all the pa-rameters for each set of conditions three replicates

were tested.

An addition of twelve specimens was also ma-chined for the uncorroded tests, half for fatigue andhalf for tensile studies further divided into anodizedand bare specimens.

With the goal of observing how S-N fatigue curveswould change with increasing corrosion, an extra15 samples were considered all not anodized andwashed with freshwater. These 15 test specimenswere tested at the Mechanical Testing Laboratoryof Instituto Superior Tecnico with the same testingconditions and procedure.

A total of 207 specimens were tested.

Before the beginning of the tests, the specimenswere cleaned and the ones that will be inserted in-side the salt spray chamber were masked on the gripareas with a 3MTM Corrosion Resistant Duct Tape(50mm× 50m) in order to protect these areas fromcorrosion.

The specimens were then labeled according to thedifferent combinations of exposure time (’0’,’1’, ’3’,’9’, ’15’ or ’20’), anodized or not (’A’ or ’NA’, re-spectively), the three washing methods (’B’ - nitricacid, ’C’- freshwater and ’D’- not wash) and thereplicate number (’01’, ’02’ or ’03’) (e.g. 1-NA-B-02: replicate number 2 of a bare specimen washedwith nitric acid exposed to one corrosion cycle).

3.2. Corrosion Test

To simulate a device that goes in seawater for a fewhours and then gets stored in a warehouse duringa certain period of time until it goes in seawateragain, a wet/dry cycle of an 8h wet period and a 15hdry period was executed (Figure 2). These periodsof time result in a total test duration of 460 hours(20 cycles).

In the wet period, the specimens were inside thesalt spray chamber (ACS Dry Corrosion Test Cab-inet 1200) following the ASTM B117 - StandardPractice for Operating Salt Spray (Fog) Apparatus[17] at the temperature of 35◦C ± 2◦C, with a saltsolution with 5wt.% NaCl and pH between 6.5 and7.2.

In the dry period, the specimens were inside ahumidity and temperature controlled chamber (Ar-alab Climatic Chamber Fitoclima 500 EP20). Thetemperature was 25◦C and the relative humidity(RH) was 80% to simulate a warehouse near thesea.

In all extracted specimens, with a MITUTOYOSurface Roughness Measuring Tester, a roughnessprofile was measured. From this profile, with anMS Excel Macro, designed purposely for the task,the maximum valley depth, average valley depth,and valley density were obtained. A microscopi-cal observation (Nikon ECLIPSE MA 100) of everyspecimen was also performed and some photographs

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(Nikon NIS Elements D software) were taken of pits,cracks and other interesting microstructures.

Figure 2: Scheme of the Test Procedure

3.3. Tensile Test

The test was performed following the standardASTM B557M - Standard Test Method for Ten-sion Wrought and Cast Aluminum and MagnesiumAlloy Products (Metric) [16]. The machine usedwas an MTS Servohydraulic Testing Machine witha load cell of 100kN and an integrated Linear Vari-able Differential Transformer (LVDT) with a de-fined rate of displacement of 1mm/min. During theexperiment, an MTS 632.85 Biaxial Extensometerwas mounted on the specimens to measure axial andtransverse deflections.

3.4. Fatigue Test

The test was performed following the ASTM E466 -Standard Practice for Conducting Force ControlledConstant Amplitude Axial Fatigue Tests of MetallicMaterials [18].

The machine used in this experiment was theMTS Servohydraulic Testing Machine with a loadcell of 50kN and an integrated LVDT. The test con-ditions were SR=0.1, f=10Hz and a maximum stressof 165.5MPa. The test limit is 1 million cycles. Forthe S-N curve specimens, the maximum stress isvariable but the other conditions are the same.

4. Results4.1. Corrosion Test ResultsThe surface evaluation revealed that the initiationof pits is confined to very few sites although theystarted to appear already on the first cycle.

The specimens washed with nitric acid (AB andNAB) were the more degraded, with the biggest anddeeper pits. In the anodized case, the acid dissolvedthe aluminum oxide that constitutes the oxide layer,discoloring the specimens. This brutal discolorationhappened after the very first wash and after that,the color continued to go back to the specimen’soriginal color, before anodization. The initial cir-cular pits started to merge with close neighboringpits preferably along the grain direction. A largepit found in an anodized washed with nitric acidsample, after 20 cycles of exposure, with an approx-imate area of 0.256 mm2, is shown in Figure 3.

Figure 3: Microscopic Image of a 20AB Specimen

Around the perimeter of this pit, there are pitlines coming out meaning the continuous propaga-tion of the pit. These pit lines initiated on the edgesof the bigger pit and are growing outwards. This isone of the aspects that decreases fatigue life becauseit covers the crack initiation stage and even a littlebit of the crack propagation period of the fatigueprocess.

In the specimens washed with freshwater, it canbe seen a breakdown of the anodization layer and asmall discoloration as the corrosion time increases,in the anodized samples. However, after the 20 cy-cles, the specimens did not present any significantdamage, which means that the anodization pro-tected the metal surface effectively. As for the notanodized samples, the crackle effect of the break-down of the natural oxide layer became more evi-dent. Pits appeared, with different sizes, all overthe exposed surface and also coalesced with closepits. Even though these specimens were washedwith freshwater they were dirty, with significant saltresidues but not as much as the non-washed.

The non-washed specimens had a similar behav-ior to the washed with freshwater either in the an-odized case or the bare case. The anodized spec-

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imens did not have many salt depositions on thesurface but had a small discoloration and an in-creased density of cracks on the layer with increasecorrosion. On the bare samples, the breakdown ofthe oxide layer started with a fish scale pattern andthen changed to a crackle effect. Pits appeared andas they grow also new ones show up.

All the samples were more corroded on the edgesthan on the middle part of the exposed surface.

The roughness profile analysis exhibit a generalincrease of maximum valley depth and average val-ley depth for all specimen variations and an oscillat-ing valley density. The more corroded samples (ABand NAB) have the highest maximum and averagevalley depth, and the least corroded (AC and AD)have the lower values. As for the valley density, theoscillating variation is because in the beginning pitsstarted to appear in a disperse way increasing thevalley density, but after this, they started to get to-gether making one larger pit which means that thedensity decreases.

4.2. Tensile Test ResultsThe evaluated parameters from the tensile testswere the ultimate tensile strength, yield stress,Young’s modulus, elongation at fracture and reduc-tion area.

Before corrosion, the main difference between an-odized and bare specimens in terms of the mechani-cal properties considered are the ductility measures(elongation at fracture and reduction area). Sincethe anodic film is brittle the ductility is slightlyless when comparing anodized with untreated spec-imens.

Almost all of the properties decreased with in-creasing corrosion exposure time as we can ob-serve in Figure 4, the lines get lower (lowering ul-timate tensile strength and yield stress) and theyalso get shorter (lowering the strain at fracture),the Young’s modulus is practically the same (theslope of the elastic area is almost the same in allcases).

Figure 4: Engineering Stress-Strain Curve Evolu-tion for AB Specimens

Ultimate and yield stress followed a liner decreas-ing trend, in general; Young’s modulus did not havea significant variation; elongation at fracture and re-duction area decreased exponentially to extremelylow final values.

4.3. Fatigue Test ResultsDue to the anodic film, there was a decrease of91.1% in fatigue life just by anodizing the speci-mens.

It is generally accepted that reduction in fatigueperformance of anodic oxide coated specimen is di-rectly related to the brittle and porous nature ofthe coating layer and tensile residual stress inducedduring coating process [9]. The thicker the coat-ing the more irregularities it contains which alsocontributes to the decrease in fatigue life. Increas-ing the thickness also causes a larger area for microcrack growth and coalescence.

For thicker oxide layers (for example hard an-odization) the crack formation can also be at-tributed to the difference in thermal expansion co-efficients between the aluminum substrate and thecoating layer along with internal tensile residualstresses in the coating [9].

As for the evolution of fatigue life with increasingcorrosion cycles, there was a general decreasing ex-ponential trend. The not anodized specimens hada significant loss in fatigue life with just one cycleof exposure which means that the smallest amountof corrosion reduces a lot fatigue life. After the firstcycle, the differences were not so accentuated but itstill decreased.

Concerning the anodized samples, at first, sincethe whole corrosion process dissolves the anodiclayer, there is almost a maintenance of fatigue lifeor even a slight increase in the first cycle. Afterthat between the continuous dissolution of the filmand simultaneous corrosion, fatigue life decreases.This is more noticeable on the specimens washedwith nitric acid (AB) (due to the almost completedissolution of the anodic layer) but it also occurson the other anodized samples.

Even though the not anodized started with higherfatigue lifetimes after twenty cycles they ended upwith less fatigue life than the anodized samples.This is because the anodization prevented the pitformation where the fatigue cracks tend to initiatefrom which means that pitting reduces fatigue morethan anodization on its own does.

Pits are considered stress concentrations fromwhere the failures occur. Fatigue cracks can nucle-ate from these corrosion pits and grow at an acceler-ated rate in a corrosive environment. The reductionin fatigue life present in every test specimen is pri-marily a result of premature crack initiation causedby pitting. The pits create an accumulation of lo-cal irreversible plastic deformation and microscopic

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flaws that grow and coalesce with other flaws fromother microscopic cracks.

Figure 5: Specimen’s Cross Section After FatigueTest [Courtesy of CEiiA]

Figure 5 shows a cross section of a selected speci-men after a fatigue test. This specimen had severalinitiation points. As the fatigue test runs cracksstarted to appear, preferably where pits were, onboth of the surfaces. The circle in Figure 5 high-lights two pits that got deeper along the cyclic load-ing. Typical fatigue benchmarks are also visibleon the lighter and softer areas and then the catas-trophic failure is represented by the rougher anddarker area.

In Figure 6 there are three S-N curves for a dif-ferent number of corrosion exposure cycles of a notanodized washed with freshwater sample, and thefirst observation made is that corrosion has a ten-dency to decrease fatigue performance.

Figure 6: Effect of Corrosion on the S-N Curve of7075-T651 Aluminum Alloy

Comparing the blue line (uncorroded specimens)with the red line (specimens that were exposed tothree cycles of corrosion), it can be seen that forthe same approximate interval of fatigue life themaximum stress of the cyclic load applied on thematerial is considerably less. The maximum stressfor the approximate same amount of cycles to fail-ure decreased around 100MPa. As for the greenline (specimens that were exposed to fifteen corro-sion cycles), when compared to the red line, there

was a decrease in fatigue cycles for the approximatesame amount of maximum stress applied. The samething is concluded: for small amounts of corrosionbig variations of fatigue life will be observed butas the corrosion exposure continues the variationsstart to decrease. For an increase in corrosion time,it is expected that the curve will move downwardsand leftwards.

5. Finite Element AnalysisBy way of comparison with the experimental re-sults, a tensile test with a finite element modelwas simulated on a test specimen. After achievingsimilar results in both methods a practical appli-cation was created that characterizes the materialbased on the degradation equations found experi-mentally. This program uses the experimental re-sults to characterize the corroded material and gen-erates the necessary material properties in the finiteelement solver standard language. These propertiescan then be used for any simulation in any struc-ture made out of 7075-T651 aluminum alloy, withinthe test domain.

5.1. SoftwareThe software used for pre-processing was Altair Hy-perMesh, the solver was MSC Nastran and for post-processing was Altair HyperView. The 3D designof the specimen was made in CATIA by DassaultSystmes R©. The program uses a combination ofMATLAB R© and MSC Nastran.

5.2. ModelShell elements (PSHELL) with quadrilateral plateelement type (CQUAD4) were used for the mesh ofthe specimen’s geometry. After a mesh convergencestudy, a 3t mesh dimension was defined with 84CQUAD4 elements and 1 RBE2 element. This givesa total of 117 nodes (Figure 7).

Figure 7: Meshed Specimen

Constraints are applied on the left grip of themodel blocking all the degrees of freedom. A loadis applied to the independent node of the RBE2 onthe other grip (Figure 8).

5.3. AnalysisA linear static analysis was made in order to vali-date the model with the material properties of thealuminum alloy 7075-T651 uncorroded and not an-odized. This analysis was made on a point from astress-strain experimental curve. The results vali-dated the model and are presented in Table 1.

Since the major changes occurred in the plasticpart of the stress-strain curve, a material nonlin-

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Figure 8: Load and Boundary Conditions Appliedon the Specimen

Table 1: Stress and Strain Value Comparison forLinear Static Analysis

Stress [MPa] Strain [mm/mm]Experimental 138.011 0.002012Numerical 138.016 0.002012

Relative Deviation 0.004% 0.00%

earity must be considered. Therefore, a non-linearapproach was also investigated.

The same elastic point was tested and the resultswere accurate (Table 2). A plastic point was alsoinvestigated (Table 3) and the nonlinear model wasvalidated.

Table 2: Stress and Strain Value Comparison of aLinear Point in Nonlinear Static Analysis

Stress [MPa] Strain [mm/mm]Experimental 138.011 0.002012Numerical 138.016 0.002000

Relative Deviation 0.004% 0.06%

Table 3: Stress and Strain Value Comparison of aNonlinear Point in Nonlinear Static Analysis

Stress [MPa] Strain [mm/mm]Experimental 541.019 0.02138Numerical 540.995 0.02131

Relative Deviation 0.004% 0.33%

5.4. Practical ApplicationAfter achieving similar results in both numericaland experimental methods a practical applicationwas created that characterizes the material basedon the degradation equations found experimentally.

When comparing the experimental tensile testcurves along the exposure cycles (Figure 4) it canbe observed that the slope until yield stress (theyoung modulus) is almost unchanged. The differ-ences between the curves rely on the plastic part ofthe graph. Because of this, the curve was dividedinto two lines: one line from the origin to the yieldstress (elastic line) and another from the yield stressuntil fracture (plastic line). This plasticity model isthe simplest one MSC Nastran uses.

The slope of the uniaxial stress-strain curve inthe plastic region is known as tangential modulus

(ET ) and a linear approximation of this line can bemade in the format y = mx + b. With the valuespresent in the experimental tensile curves, a linearapproximation of the plastic line was made for thedifferent experimental curves. The effect of corro-sion had an exponential trend on the slope (m) anda linearly decreasing trend on the y-intercept (b) forall the different combinations of bare and anodizedaluminum with the three washing methods. Themore corroded cases (AB and NAB) had the higherincrease of m and the higher decrease of b.

Having the knowledge of how the yield stress, ul-timate stress and slope and y-intercept of the plasticline degrade, the material properties for any corro-sion exposure time can be achieved.

Figure 9: Program Scheme

MATLABR© code number one (Figure 9) readsthe desired condition code and the number of cor-rosion exposure cycles and, with the formulas de-duced from the experimental curves, obtains thevalues of yield stress, ultimate tensile strength andm and b of the material’s plastic line. Inside thispart of the executable, there is a pre-made Nastrancard of the specimen model during a tensile testwith the nonlinear analysis model. This .bdf file al-ready contains all the mesh characteristics, materialproperties, forces, and constraints applied and de-fault nonlinear parameters so that a nonlinear staticanalysis (SOL 106) can be performed.

With the ultimate stress and area values, themaximum force is calculated which is further di-vided into 20 force values equally spaced, so that astress-strain curve can be constructed. After this,MATLABR© generates 20 .bdf files with the up-dated material properties (Nastran bulk data en-tries: MAT1, MATS1, and TABLES1) and writes,in each one, a different force value (Nastran bulkdata entry: FORCE).

From this first code are also created two .txtfiles. The first file (Output.txt) contains the tensilestrength and yield stresses of the corroded materialas well as the Nastran code lines that need to bechanged concerning the material properties (MAT1,MATS1 and TABLES1). The second file (Data.txt)contains the data from the uncorroded stress-strain

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curve (contained in a vector in code one dependingif the user chooses an anodized or a bare material)and the interpolated curve constructed based on theplasticity theory.

The next step is to run the 20 .bdf files in Nastranwhich generates 20 .f06 files. Since it is only neededone value for stress and strain, one element fromthe mesh had to be chosen. In tensile tests, the”dogbone” specimen shape is the most typical shapeso that the deformation is confined to the narrowcenter region to reduce the likelihood of fracture tooccur at the ends of the specimen. Because of this,in the input file, the requested outputs were onlythe stress and the strain present in the element inthe middle of the specimen, being this region themost probable for fracture to occur.

The second MATLABR© code reads the resultsof the finite element analysis as well as the datafrom the Data.txt file. With this values, it plotsa graph comparing three stress-strain curves: theuncorroded experimental, the interpolation basedon the plasticity approach considered and the FEMresult.

6. Conclusions• Corrosion exposure to salt fog leads to exten-

sive pitting of the not anodized material sincethe first cycle, as expected;

• Increased corrosion was present on the edges ofthe exposed surface;

• Anodization protected the metal effectivelyfrom corrosion;

• Maximum and average valley depth both in-creased with exposure time. Valley density hada lot of variations but with small amplitude;

• Reduced cross-section and stress concentra-tions caused by corrosion greatly influenced theload carrying capacity of the specimen reduc-ing, in general, ultimate and yield stress at aconstant rate;

• Corrosion-induced degradation of mechanicalproperties occurs gradually for every tensilecondition tested. Young’s modulus did nothave a significant variation. Elongation at frac-ture and reduction area decreased exponen-tially to extremely low final values;

• From the tensile tests, the ductility measureswere the more affected properties with higherdegradation percentage. Corrosion increasedthe material’s brittleness which in turn canchange the failure mode to a much more dan-gerous brittle failure;

• Corrosion had a greater degradation effect onthe ultimate tensile strength than on the yield

stress due to the fact that corrosion affectedmostly the plastic domain of the material;

• Based on experimental observations, fatiguecracks initiate from corrosion pits, as predicted;

• Fatigue performance of 7075-T651 alloy wassignificantly reduced by the anodic oxidationprocess. The degrading effect was about 91.1%reduction. This reduction can be primarily as-cribed to deep micro cracks formed during theanodization process;

• Fatigue life appears to follow an exponentialreduction with increasing exposure time;

• Small amounts of corrosion decrease the notanodized specimens fatigue life significantly;

• The fatigue S-N curve moved down and leftwith increase in corrosion cycles. This meansthat corrosion reduces the number of cycles tofailure for the same maximum load applied;

• In a long term perspective fatigue life is morereduced by corrosion on an untreated specimenthan on an anodized, which makes the treat-ment an advantage for long term applications;

• The freshwater washing method made a morepositive difference, in general, on the not an-odized than on the anodized samples;

• Specimens tend to fail on the more corrodedareas either in tensile or fatigue tests as antic-ipated;

• In a general balance, the specimens that pre-sented the best behavior were the anodizedwashed with freshwater and not washed. Ifthey resisted the salt spray exposure they willdefinitely resist seawater and sea atmospheresince the test is more aggressive than reality(conservative approach);

• This results correspond to the aluminum al-loy 7075-T651 and cannot be extrapolated toother alloy compositions nor other temper des-ignations.

The following recommendations are suggested forfurther investigation:

• The lack of a quantitative correlation betweenaccelerated laboratory corrosion tests and in-service corrosion attack or atmospheric corro-sion tests calls for additional investigation re-lated to corrosion of aluminum structures;

• Investigating other types of corrosion tests, forexample, immersion tests may produce lessdamaged metal and more accurate results;

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• Since the device, where the material will beused, works in high pressure environments aninfluence of pressure in corrosion should be fur-ther evaluated;

• Some metallographic cross sections of the an-odized specimens could be prepared to analyzethe loss in thickness of the oxide layer alongthe cycles of exposure;

• To investigate the influence of corrosion on theelastic domain the test can be repeated withmore corrosion cycles and more replicates foreach case scenario;

• The plasticity model used can be approachedwith more accuracy. Suggestion: In this thesis,just two lines are defined to describe the stress-strain curve, however, this number could beincreased, especially in the plastic zone.

AcknowledgementsThis research was supported by CEiiA, a Por-tuguese center of engineering and product develop-ment. The author acknowledges B. Albuquerque,T. Rebelo, P. Figueiredo, C. Silva, S. Sampaio, J.Torrejon, M. Oliveira and I. Martins of CEiiA andProfessor L. Reis of IST, University of Lisbon.

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