In this article, the corrosion behavior of commercial AZ31 welded plates in aqueous chloride media was investigated by means of gravimetric techniques and Neutral Salt Spray tests (NSS). The AZ31 samples tested were welded using Gas Tugsten Arc Welding (GTAW) and different filler materials. Material microstructures were investigated by optical microscopy to stablish the influence of those microstructures in the corrosion behavior. Gravimetric and NSS tests indicate that the use of more noble filler alloys for the sample welding, preventing the reduction of aluminum content in weld beads, does not imply a better corrosion behavior.
Magnesium and its alloys constitute from the point of view of light alloys a very interesting group of materials. Their lightness represents a major advantage in structural applications, but it is not the only one; the mechanical properties at high temperature-fatigue strength, dimensional stability, and alkaline resistance - in comparison with plastic materials (Avedesiam and Baker,
Magnesium is a very electronegative material which can suffer different corrosion processes such as galvanic corrosion, pitting corrosion, filiform corrosion, stress corrosion cracking, fatigue corrosion and according to some investigators, also intergranular corrosion (Zeng
In aqueous environments, the anodic and the cathodic reactions commonly accepted (Pardo
Mg→Mg+2 + 2e− (1)
2H2O+2e−→H2 +2OH− (2)
In this article, the corrosion behavior of magnesium alloys type AZ31 samples welded by means of Gas Tungsten Arc Welding (GTAW) in salt aqueous solutions is investigated, trying to clarify from an experimental point of view its corrosion resistance and to find numeric relations between the welding parameters and the corrosion rates for the different samples.
The AZ31 alloy is a wrought alloy 3% of aluminum, 1% zinc which is considered to have a good relative weldability (Avedesiam and Baker,
Magnesium and its alloys can be joined by stud welding, spot welding and other resistance-welding processes as well as electron-beam, friction, explosion, and ultrasonic. Recently there have been several papers investigating the use of laser (Zemin
However arc welding-inert gas methods are the most commonly used (Avedesiam and Baker,
For all those reasons GTAW using high purity argon was chosen as welding method. GTAW is a welding system which allows to obtain high quality weld joints while minimizing spots, arc instability, porosity and the lack of penetration that may happen in the case of other arc welding techniques such as Shielded Metal Arc Welding (SMAW) or laser (Ben-Hamu
Some studies suggest that the corrosion of magnesium alloys in aqueous media may be explained considering three different factors: the chemical composition of matrix phase α, the composition of the other phases present in the alloy and how those phases are distributed in the matrix (Ming-Chun
In relation to the previous factors involved in the corrosion behavior, there are several publications about these alloys in which it has been studied by means of gravimetric (Walton
For this purpose it was decided to use gravimetric techniques instead of electrochemical ones considering the difficulty to measure corrosion in a reliable way indicated by other authors (Shi
The structure of the samples has been studied using optical microscopy, trying to identify the different phases involved in the welding of the AZ31 alloy, as mentioned before this is not an easy task especially if the difficulties found out by other authors are considered.
The object of this study is to define the corrosion behavior of welded joints, to determine by means of metallographic techniques the structures and the grain size of the different welding areas and to quantify the corrosion rates of the samples considering the filler metals used.
In this study different AZ31B alloy welded samples with different filler metals were used. The welded samples were plates of 200×150×3.2 mm, for the welding of the samples Gas Tungsten Arc Welding (GTAW) system with alternate current was chosen. Samples were welded using one pass in the front and one in the back, using two current levels, 120 A for the front pass and 80 A for the back pass. The voltage was around 17 V and the speed for each pass was around 3×10−3 m s−1.
As filler metals AZ31 and AZ92, 4 mm diameter rods were used, according to Avedesiam and Baker (
To carry out the tests, 50×12×3.2 mm samples with 1.6×10−3 m2 area surface were taken, according to the sample size used by other authors (Pardo
In this research AZ31 alloy base metal samples get from non-welded plates and transversal samples of the welded joints, including the melted joint, heat-affected zone and base metal, were used.
The electrolyte used during testing was an aqueous solution of NaCl with the concentrations indicated in next chapter.
In this study two different gravimetric techniques were used. The first was an standard Neutral Salt Spray (NSS) tests according to international standard ISO 9227 (AENOR
The immersion test were carried out by immersing in 0.75 liters of NaCl 3.5% weight solution the samples, the mass of each sample was measured with a precision micrometric scale before testing.
For the analysis of the different factors and parameters which could influence in the corrosion behavior, corrosion tests were done using groups of three samples in order to discriminate the significant effects. The next parameters were considered as basis of the tests:
Filler metal (AZ31/AZ92)
Agitation of the electrolyte
Relative surface of the welding beads and the base metal of the samples.
The tests were carried out in two groups with different test durations, the first one up to 496 hours, the second one up to 1200 hours (50 days). For the first testing group, the effect of the electrolyte movement on the corrosion rate was studied.
In the case of the salt spray test, the chamber and the procedures described for Neutral Salt Spray tests in the international standard ISO 9227 (AENOR
In these tests 12 samples similar to the used in the previous gravimetric tests were used. The values of the collection rates were kept within the limits allowed by the ISO standard 9227(AENOR
The tests were carried out during 15 days, measuring the mass of the samples at their end.
All samples were weighted before and after the test. Because of the thick layer of corrosion products appeared during the test, in order to evaluate the corrosion rate and the mass loss during NSS tests it was necessary to immerse the samples in chromic acid H2CrO4 to dissolve it.
The samples were cut, polished and etched with acetic-picral (10−2 L acetic acid; 4.2 g picric acid; 10−2 L H2O and 7×10−2 L ethanol) during 30–40 seconds, then washed with ethanol and dried with a blast of hot air.
The granulometry and micrographic study shows that the structure of the material as fabricated (hot rolled) presents fine grain size G9 in accordance with ISO-643 (AENOR
a) AZ31B plate micrography (X500); b) detail of a cross section welding joint, the heat-affected zone and the melted material (X200). Detail of MnAl2 particles is included (identification according to Pardo
a) Base metal (X200); b) base metal detail. Twins can be seen inside the crystals. (X1000). β-phase precipitated particles are indicated (according to Avedesiam and Baker,
Weld beads have a dendritic structure with a grain size slightly larger, between G=7.5 and G=6 units and as mentioned before, precipitate particles of β-phase Mg17Al12 or Mn-Al compounds are observed spread in the α-matrix phase (
a) Detail of the HAZ compared with welded metal, upper-left corner, and base metal, down-right corner of the image (X100); b) grain size G of the samples according to the ISO-643 standard. Filler material is indicated for welded samples.
In the case of the Heat-Affected Zone (HAZ) its grain size varies between G=7 and G=3, grain size is larger in the HAZ of welded samples using AZ92 as filler metal (
Comparative of the corrosion rate for electrolyte within and without agitation.
The study of corroded surfaces shows that the attack takes place in a section close to the HAZ, it starts as localized corrosion in the areas of larger grain size (corrosion resistance of AZ31B alloy tends to increase as grain size is reduced), probably due to the enhanced passivity of surface oxide films (Liao
As previously stated, two different test rounds were done, the first one up to 406 hours, and the second one up to 1200 hours (50 days).
Immersion tests indicate the influence of the electrolyte agitation in corrosion rates. Values for samples in electrolyte with agitation are between 0.35 and 0.51 mm/year meanwhile in tests carried out without agitation those values vary between 0.22 and 0.3 mm/year as indicated in graphic shown in
Mass loss per surface unit as function of time (hours). First round test.
Polynomic kinetic laws representing mass loss per surface unit (y, expressed at mg cm−2) as function of time (x, expressed at hours)
y | y=c·x2+a·x+b | SAMPLE DESCRIPTION | ||||||
---|---|---|---|---|---|---|---|---|
c |
a |
b |
R2 |
Name | Filler metal | Sample area | Description | |
−8 | 1.09 | 2.64 | 0.999 | B1 | − | Base metal | Agitated electrolyte | |
−4 | 1.04 | 7.79 | 0.996 | B2 | − | Base metal | Agitated electrolyte | |
−7 | 1.41 | −15 | 0.998 | B3 | − | Base metal | Agitated electrolyte | |
1 | 0.37 | 11.62 | 0.992 | B4 | − | Base metal | Agitated electrolyte | |
0.8 | 5.80 | 6.88 | 0.998 | B5 | − | Base metal | Agitated electrolyte | |
1 | 0.54 | 3.87 | 0.998 | B6 | − | Base metal | Agitated electrolyte | |
−8 | 0.90 | 7.53 | 0.992 | B7 | − | Base metal | Agitated electrolyte | |
0.2 | 0.38 | 7.69 | 0.994 | MT | AZ31B | Cross section sample | Agitated electrolyte | |
−6 | 0.90 | 10 | 0.996 | TT | AZ92A | Cross section sample | Agitated electrolyte | |
−4 | 0.64 | 1.11 | 0.987 | B8 | − | Base metal | Agitated electrolyte | |
|
||||||||
−3 | 1.08 | 39.44 | 0.987 | M3A | AZ31B | Cross section sample | Agitated electrolyte | |
−7 | 1.87 | 1150.3 | 0.984 | T3B | AZ92A | Cross section sample | Agitated electrolyte | |
−1 | 2.75 | 122 | 0.99 | MBB | − | Base metal | Agitated electrolyte |
quadratic value coefficient of the polynomic expression.
lineal value coefficient of the polynomic expression.
constant value coefficient of the polynomic expression.
Regression coefficient.
During the second test round, the behavior of the samples was coherent with the previous results; again parabolic mathematical laws were obtained.
Mass loss per surface as function of time graphic. Second round test.
NSS test were carried out during 16 days.
a) Mass loss in NSS test; b) samples after NSS test.
The resultant micrographies indicate that the cross sections of the sample plates show an equiaxic structure (see
In samples welding areas precipitated particles are observed all over the melted region. Those particles are spread along the α-matrix and in the grain boundaries without preferential precipitation areas. Grain size is slightly larger than the observed in the base metal, this fact is coherent in welding processes (
The growth of the grain in the HAZ shown in
The obtained results in the NSS tests show that corrosion resistance is affected by welding processes, as can be seen in
During immersion tests, welded samples shown corrosion rates lower than the obtained for base metal (see
Immersion tests show parabolic laws to represent the corrosion rate as function of time. This behavior is probably related with the formation of protective corrosion layers, further investigation is needed about this hypothesis. For all tests the determination coefficients are close to 1, being better than in the case of linear approximations. However, as indicated in
Corrosion rates of AZ31B filler metal welded samples are smaller than those obtained for AZ92 filler metal welded samples (
Different welding processes do not imply a significant modification of AZ31B corrosion resistance in NaCl solutions if those welding's are done within right and qualified procedures. Obtained microstructures for welding processes must be taken into account as a major parameter in corrosion resistance. The use of noble filler materials (including higher content of aluminum) does not guarantee an improvement of welding joint corrosion resistance because anodic and cathodic zones can be formed reversing the improvement of a better chemical composition.
The authors would like to acknowledge the financial support by the Dpto. de Física Aplicada e Ingeniería de Materiales, E.T.S. Ingenieros Industriales de Madrid, Universidad Politécnica de Madrid.