Tartaric-sulfuric Acid Research Articles

Incorporation of Anions into Anodic Alumina-A New Track in Cr(VI) Anodizing Substitution?

Aluminum technical alloys are well known for their outstanding mechanical properties, especially after heat treatment. However, quenching and aging, which improve the mechanical properties, by the formation of Cu-rich zones and phases that are coherent with the matrix and block the dislocation motion, cause uneven distribution of the elements in the alloy and consequently make it prone to corrosion. One method providing satisfactory corrosion protection of aluminum alloys is anodizing. On an industrial scale, it is usually carried out in electrolytes containing chromates that were found to be cancerogenic and toxic. Therefore, much effort has been undertaken to find substitutions. Currently, there are many Cr(VI)-free substitutes like tartaric-sulfuric acid anodizing or citric-sulfuric acid anodizing. Despite using such approaches even on the industrial scale, Cr(VI)-based anodizing still seems to be superior; therefore, there is an urge to find more complex but more effective approaches in anodizing. The incorporation of anions into anodic alumina from the electrolytes is a commonly known effect. Researchers used this phenomenon to entrap various other anions and organic compounds into anodic alumina to change their properties. In this review paper, the impact of the incorporation of various corrosion inhibitors into anodic alumina on the corrosion performance of the alloys is discussed. It is shown that Mo compounds are promising, especially when combined with organic acids.

The Corrosion Resistance of Tartaric-Sulfuric Acid Anodic Films on the 2024 Al Alloy Sealed Using Different Methods

Tartaric-sulfuric acid anodic (TSA) films were prepared on the surface of the 2024 Al alloy. These films were sealed with cerium salts at 25 °C and 65 °C, hot water, and dichromate. The morphology and corrosion resistance of the anodic films were investigated using a field emission scanning electron microscope/energy-dispersive spectrometer, an electrochemical workstation, an acidic spot test, and an immersion test. The results indicated that the surface of the TSA film sealed with cerium salt at 65 °C had a slightly lower cerium content compared to the TSA film sealed at 25 °C. It was found that increasing the sealing temperature of cerium salt could enhance the corrosion resistance of the TSA film. After immersion in a 3.5 wt.% NaCl solution for 336 h, no obvious corrosion pits were observed on the surface of the TSA film sealed at 65 °C, whereas many larger corrosion pits appeared on the surface of the TSA film sealed at 25 °C. The improved corrosion resistance of the TSA film sealed at 65 °C could be attributed to the synergistic effect of cerium oxide deposition and the hydration reaction. The corrosion resistance of the TSA film sealed at 65 °C was significantly better than that of the film sealed with hot water, but it was still lower than that of the TSA film sealed with dichromate.

Addition of molybdate ions to the anodizing bath to improve the corrosion resistance of clad 2024-T3 alloy anodized in tartaric-sulfuric acid

In the present investigation molybdates ions (MoO4−2) were added to a tartaric‑sulfuric acid (TSA) bath, which was employed to anodize AA2024-T3 clad samples. The incorporation of Mo into the anodized layer was investigated by different techniques: X-ray photoelectron spectroscopy (XPS), glow-discharge optical emission spectroscopy (GDOES), Rutherford backscattering spectrometry (RBS) and scanning electron microscopy (SEM), while the corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) techniques in neutral (0.5 M) and acidic (0.1 M, pH = 4.0) NaCl solutions. The different characterization techniques indicate that small amounts of the modifier were incorporated into the anodized layer, while the SEM analysis confirmed that the morphology of the layer was not changed, although it was thicker when anodized in the Mo containing bath. Evaluation of corrosion resistance by EIS in both media showed that samples anodized in the molybdate containing bath were more corrosion resistant and durable. Fitting results of the electric equivalent circuit indicate that this may be a consequence of a thicker barrier layer for these samples.

Chromium/nickel-free conversion coating as cold post-treatment to anodized AA2024-T3 for corrosion resistance increase

The AA2024-T3 alloy exhibits significant properties for industrial applications, including low weight and high mechanical strength. However, its susceptibility to corrosion, attributed to precipitation hardening, requires effective surface treatments. Anodized aluminum alloys offer a robust protective structure; nevertheless, their porous layer must enable the anchoring of an additional coating or be adequately sealed. Boiling water sealing, though a non-toxic process, demands substantial energy consumption, while toxicity concerns limit the use of sealing with dichromate and Ni salts. Consequently, there is a pressing need to explore novel sealing methods capable of operating at lower temperatures for brief durations without Cr(VI) or Ni. This study proposes an alternative post-treatment for AA2024-T3 anodic layers generated in tartaric sulfuric acid (TSA). The method involves immersing the samples for 5 min in solutions containing H2ZrF6 alone, Na2MoO4 with KMnO4, and a combination of all three. The resulting layers underwent analysis using scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), open circuit potential (OCP) measurements during coating deposition, electrochemical impedance spectroscopy (EIS), and a visual examination post-immersion in a NaCl solution. The findings indicate that the Zr and Zr-Mo-Mn conversion coatings partially sealed the porous structure of the anodic oxide film, thereby enhancing its corrosion resistance in chloride medium. The incorporation of molybdate and permanganate ions intensified the precipitation of Zr oxide-fluorides within and on the porous layer, leading to improved corrosion resistance of the anodized alloy compared to Zr-only coating.

An assessment of pitting corrosion in anodized aluminum alloys: It might not be what it seems

AbstractIn this study, special attention was given to the characterization of pits on the anodized aluminum alloy (Al–Zn–Mg–Cu) with an anodic aluminum oxide formed in tartaric sulfuric acid. Anodic polarization in 0.1 mol L−1 NaCl solution was used to initiate pitting corrosion in the anodized alloy. Pit characteristics, such as morphology and depth, were evaluated by using optical microscopy and optical profilometry, and scanning electron microscopy. The methodology adopted in this study revealed severe under‐film pitting due to highly occluded conditions and showed that the extent of the under‐film pitting is significantly greater than the size of pit mouth observed from the surface.

Morphological and semiconductive properties of the anodic oxide layers made on Fe3Al alloy by anodizing in tartaric-sulfuric acid mixture

It has recently been found that the anodizing of FeAl alloys allows the formation of iron-aluminum oxide layers with interesting semiconducting properties. However, the lack of systematic research on different anodizing regimes is hampering their full exploitation in numerous photoelectrochemical-related applications. This study address, for the first time, the systematic effect of the electrolyte composition on the formation of self-ordered oxide films by anodizing on cast Fe3Al alloy. The Fe3Al alloy was anodized in 3 electrolytes with different water-ethylene glycol (EG) ratios (pure water, 25 vol.%-EG, and 50 vol.%-EG solutions) at a constant tartaric-sulfuric acids concentration, different voltages (10–20 V) and treatment times (2–60 min). After anodizing, all anodic oxide layers were annealed at 900 °C to form semiconductive iron-aluminum crystalline phases. Conventional techniques were used to systematically ascertain the morphological (SEM/EDS, XRD, eddy-current measurements) and semiconductive (UV–VIS reflectance spectroscopy) properties of these oxide layers. The results confirmed the formation of homogeneous and self-ordered anodic oxide layers at 10 and 15 V, regardless of the electrolyte composition. Namely, anodic films formed in electrolytes containing EG showed lower pore sizes, growth rates, and film thicknesses than those anodic films formed in the aqueous-based electrolyte. The annealing post-treatment results in different Fe-Al oxides (FexOy, FeAl2O4, etc.) with superior band gap values than those for non-annealed films.

Influence of the Anodizing Time on the Microstructure and Immersion Stability of Tartaric-Sulfuric Acid Anodized Aluminum Alloys

Tartaric-sulfuric acid anodizing (TSA) has been selected by the aerospace industry to replace Cr(VI)-based anodizing treatments of aluminum alloys. Modification of the bath composition can result in the necessity to revise the process conditions, including the time necessary to obtain the desired properties of the anodized layer. This study focuses on the microstructure and immersion stability of the pilot scale anodized AA2024 aluminum alloy, with anodizing times of 25 min and 45 min. The layer structure was characterized by scanning electron microscopy (SEM) and glow discharge optical emission spectrometry (GD-OES). The electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion resistance and immersion stability of the samples in 0.01 M NaCl solution. The density of pores formed in the 45 min anodized samples was higher which correlated with higher impedance modulus at equivalent immersion time and higher thickness of the oxide layer. Contact angle measurements demonstrated better wettability of the 45 min anodized sample. The results suggest that 45 min anodizing offers higher corrosion resistance and better initial adhesion with subsequent post-treatment such as sealing or painting.

An ORP-EIS approach to distinguish the contribution of the buried interface to the electrochemical behaviour of coated aluminium

In this work, the influence of the buried coating/metal oxide interface as a crucial parameter in hybrid systems’ durability was investigated. To evaluate these hybrid interfaces in situ and in realistic conditions/environments, an integrated approach based on the use of odd random phase electrochemical impedance spectroscopy (ORP-EIS) is employed. In this approach proposed previously by our group, in addition to the effectively shortened measurement time, supplementary information about the stochastic noise, non-linearities and non-stationarities is provided. This information allows the rapid analysis of the error structures on the experimental data as well as providing a statistical evaluation for the proposed electrochemical equivalent circuit (EEC). To elaborate on the interfacial influences, aluminium substrates received two different surface pre-treatments (tartaric sulfuric acid (TSA) anodizing and acidic pickling) and were coated with a pigmented polyurethane based and a pigmented epoxy-amine based coating. The ORP-EIS data was fitted to an appropriate EEC and the output parameters were used to distinguish between the diffusion of water and ions. It was shown that prior to the arrival of ions at the buried interface, the coating response is dominating. With further diffusion of ions and their arrival at the interface, the onset of the contribution of different surface pre-treatments was captured and quantified.

(Digital Presentation) The Effect of Tartaric-Sulfuric Acid (TSA) Anodizing on the Corrosion Resistance of the AA7475-T761

The microstructure of the high strength aluminum alloy, AA7475–T761, in the as-received condition was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, and statistical analyses of grain size and micrometer precipitates distribution and density were performed. An anodic aluminum oxide (AAO) film was potentiostatically grown on the alloy in tartaric sulfuric acid (TSA). The anodizing behavior of the alloy and the mechanisms of localized corrosion initiation and propagation in bare (not anodized) and anodized conditions were discussed. The alloy corrosion behavior was investigated in sodium chloride solutions using electrochemical techniques (cyclic potentiodynamic polarization) and the morphology and extent of corrosion propagation was investigated through optical microscopy, optical profilometry and scanning electron microscopy analyses. It was observed that the two alloy surface conditions (bare and anodized) play different roles in the propagation of the localized corrosion process. The bare alloy presented lower localized corrosion potentials and the propagation of the localized corrosion was shallower, but more heavily distributed, than in the anodized condition. However, the alloy in the anodized condition presented lower repassivation potentials due to the deeper pits formed. In addition, the variability of pitting potentials for the anodized condition was high, due to the heterogenous structure of the AAO film formed, and dependent on the time and drying storage conditions previous to electrochemical characterizations. The electrochemical results were correlated with the microstructural characteristics of the oxide surface film of the alloy in both conditions, bare (not anodized) and anodized.

Ce post-treatment for increased corrosion resistance of AA2024-T3 anodized in tartaric-sulfuric acid

The effect of a post-treatment for short time in 50 mM Ce(NO3)3 solution, with or without H2O2, on the corrosion of AA2024-T3 anodized in tartaric-sulfuric acid was investigated. Electrochemical (EIS, polarization curves) and corrosion (immersion) tests showed improved performance for samples post-treated at 50 °C in the H2O2 containing solution. Microstructural characterization (SEM, STEM, GDOES) evidenced the presence of Ce oxyhydroxides both at the surface and within the pores of the anodized layer, and their preferential interaction with defective sites both at micro and nanoscale. Important amounts of Ce-species were found near corrosion products, indicating active corrosion protection by Ce ions.

TSA anodising voltage effects on the near-surface coarse intermetallic particles in the AA2024-T3 and AA2198-T8 alloys

ABSTRACT In this study, the behaviour of the micrometric particles of the AA2198-T8 alloy during anodising at various voltages and the effect of anodising voltage on the anodised surface morphology have been investigated in a tartaric-sulfuric acid anodising solution. The results were compared with that of the AA2024-T3 alloy. For the AA2198-T8 alloy, partial dissolution of these particles occurred at 0, 3 and 4 V. Besides, for potentials above 5 V, there is a preferential dissolution of the intermetallic particles. For the AA2024-T3 alloy, the results indicated a total dissolution of the micrometric particles at 0 V and a partial dissolution at 3 V, whereas above 4 V total dissolution occurred. Between 1 and 2 V, no dissolution was observed for both alloys. The preferential dissolution of the micrometric particles resulted in defects in the anodic film and cavities on the anodised surfaces.

Influence of anodizing surface treatment on the aging behavior in salt-fog environment of aluminum alloy 5083 to fiber reinforced composites adhesive joints

ABSTRACT The present work aims to evaluate how an innovative and eco-friendly anodizing process with tartaric-sulfuric acid (TSA) bath and a pore widening in an aqueous solution of NaOH 0.1 M can improve the durability in marine environment of co-cured adhesive joints between an aluminum alloy (AA 5083) and fiber (i.e., glass or basalt)-reinforced composite substrates. The aging was carried out by exposing samples to salt-fog spray conditions (5 wt.% NaCl) for 30, 60 and 90 days in a climate chamber, in accordance with ASTM B117. In order to get further insights on the aging behaviour of bonded joints, neat matrix was also exposed to the same environment for 60 days. Quasi-static mechanical tests and morphological analysis through SEM microscope coupled with EDX analyser were carried out to deeper understand how the fracture surfaces change during aging and how this affected the mechanical performances of joints.

In-situ growth of cerium nanoparticles for chrome-free, corrosion resistant anodic coatings

Chrome-based anodization and sealing combined is a cost-effective solution for developing good corrosion protective coatings on aluminum (Al) and its alloys. The toxicity of hexavalent chromium, however, requires new, sustainable, environmentally friendly, and efficient chromate-free sealing procedures. Herein, a combination of cerium (Ce) sealing and hydrothermal sealing (HTS) is employed to improve the corrosion resistance of the commonly used alloy AA2024-T3 anodized by tartaric sulfuric acid (TSA). The morphologies and components of the resulting surfaces are systematically studied. Characterization technologies such as SEM, TEM, EDX, XRD, and XPS demonstrate the in-situ growth of Ce oxide nanoparticles all through the porous structures of the anodic coating and closing of the pores by additional HTS treatment. The results from the standardized corrosion test (ASTM B895) demonstrate an improved corrosion resistance obtained by the utilized chrome-free process.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Tartaric-sulphuric acid anodized clad AA2024-T3 post-treated in Ce-containing solutions at different temperatures: Corrosion behaviour and Ce ions distribution

The effect of temperature of a Ce-H2O2 post-treatment on the corrosion resistance of clad AA2024-T3 anodized in tartaric-sulfuric acid as well as the distribution of Ce oxyhydroxides in the anodized layer was investigated. Electrochemical impedance spectroscopy tests showed that samples post-treated at moderate temperatures (up to 50 °C) presented more stable and slightly higher impedance modulus than untreated ones. Increasing the post-treatment temperature to 75 °C decreased the corrosion resistance, likely due to damaging of the porous and barrier layer protective properties, as indicated by electric equivalent circuit fitting. Scanning electron microscopy characterization showed that Ce oxyhydroxides deposition (3+ and 4+ oxidation states as determined by X-ray photoelectron spectroscopy) was enhanced with increasing post-treatment temperature, and that pores were not blocked. Glow Discharge Optical Emission Spectrometry and Rutherford Backscattering Spectrometry analyses indicated local enrichment of Ce species at the bottom of the pores, whereas scanning transmission electron microscopy confirmed the presence of Ce-containing nanoparticles stuck to the pore’s walls. Analyses of corroded samples showed increased amounts of Ce oxyhydroxides on the surface and that Ce species remained inside the pores, indicating that the post-treatment protocol successfully and durably incorporated Ce ions within the structure of the anodized layer.

Assessment of the corrosion resistance of self-ordered anodic aluminum oxide (AAO) obtained in tartaric-sulfuric acid (TSA)

The corrosion performance of self-ordered porous anodic aluminum oxide (AAO) has been studied and correlated to its structure. High purity aluminum alloy (>99.999%) has been anodized using three different conditions: (a) in tartaric-sulfuric electrolyte employing common disordered conditions (TSA), (b) in sulfuric acid applying self-ordering conditions (SA), and (c) in tartaric-sulfuric electrolyte applying self-ordering conditions (TSA-SA). Some samples were post-treated in boiling water with the aim of sealing their pores. The morphology and order of the AAOs were determined by means of scanning electron microscopy and Fast Fourier Transform (FFT), while their corrosion behavior was assessed in 0.59 M NaCl employing potentiodynamic polarization tests (LP) and electrochemical impedance spectroscopy (EIS). The evolution of impedance of samples SA and TSA was monitored during 1000 h of immersion in 0.59 M NaCl. The three types of samples present similar pore diameter (Dpo) and certain degree of order, although TSA shows the lowest. However, TSA shows smaller interpore distance (Dint), thinner cell wall thickness (tw) and higher pore density (ρpo) and porosity (P) than SA and TSA-SA, which feature very similar values for these structural parameters. The LP curves do not allow to discriminate between samples. SA and TSA-SA have higher impedance than TSA among samples not sealed, probably because a thicker barrier layer forms in samples with self-ordering regime. When sealed, TSA samples show higher impedance than SA and TSA-SA in the medium frequency range because porous membranes with lower order are more easily sealed. All types of AAO, both unsealed and sealed, undergo self-sealing process during the immersion experiments. According to EIS interpretation, TSA porous oxide is self-sealed in a more efficient manner than SA and TSA-SA. These findings imply that self-ordering conditions can enhance the corrosion resistance of AAO.

Corrosion improvement of AA7075 through precipitation hardness and anodization

Cast aluminum alloy (AA7075) has been precipitation treated and anodized in Tartaric-sulfuric acid (TSA). The effects of precipitation treatment and precipitation treatment plus TSA anodization on corrosion improvement of the cast AA7075 was investigated. The microstructure and morphology of the samples were studied using metallurgical microscope and scanning electron microscope with energy dispersive x-ray (SEM EDS). Weight loss method was used to access the corrosion behavior of the alloy in 0.5 M NaCl. Results obtained indicates significant role of precipitation treatment, precipitation treatment and TSA on corrosion improvement of AA 7075. The microstructure of the precipitation treated samples in palm kernel oil and SAE 40 engine oil showed precipitates of MgZn2. The anodic film coating of the precipitation treated in palm kernel oil plus TSA anodized sample recorded highest micro hardness value of 124.33 HV, while the least value of 178.67 HV was recorded for sample precipitation treated in SAE 40 engine oil and TSA. Precipitation treated and TSA anodized samples show higher corrosion resistance improvement, with the Palm kernel oil treated recording 22.05% corrosion penetration rate while Palm kernel oil treated and TSA anodized sample recorded 17.86% corrosion penetration rate.

Sol–gel coatings with hydrothermal hydroxylation as pre-treatment for 2198-T851 corrosion protection performance

2198-T851 alloy samples were anodized in tartaric-sulfuric acid (TSA) and pretreated by hydrothermal hydroxylation, then protected by application of a hybrid sol-gel coating. The sol-gel coating was prepared using a solution obtained tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). The morphology and thickness of the film were evaluated by field emission scanning electron microscopy (FE-SEM). The corrosion resistance and corrosion mechanism of the film were analyzed by electrochemical testing and Long-term corrosion resistance testing. The results show that the sol-gel film preparing with hydrothermal hydroxylation as pre-treatment were smooth and dense. The treatment with hydrothermal hydroxylation and the hybrid sol-gel improved barrier properties and long-term corrosion resistance of 2198-T851 alloy.

Effect of Cerium Addition to a Hydrothermal Treatment on the Corrosion Protection of the Tartaric-Sulfuric Acid Anodized AA2524-T3

To protect Al alloys from corrosion, standard procedures in the aerospace industry use chromium-based acid anodizing with subsequent post-treatment steps also containing hexavalent chromium (Cr(VI)...

Corrosion resistance of tartaric-sulfuric acid anodized AA2024-T3 sealed with Ce and protected with hybrid sol–gel coating

2024 aluminum alloys are widely used in the aerospace industry due to properties as lightweight, high specific strength and durability. However, they are prone to localized corrosion due to its high amount of intermetallics. A promising method to protect the exposed metal surface is to use eco-friendly alternative corrosion inhibitors in combination with a barrier coating system. In this study, a treatment in an aqueous solution with the addition of cerium ions has been proposed and its effects on the corrosion resistance of the AA2024-T3 alloy were investigated. Samples were anodized, hydrothermally treated in aqueous solutions, containing or not cerium ions at the boiling temperature and then coated with sol-gel. The effect of Ce ions in the characteristics of the surface film formed, such as morphology and corrosion resistance, was investigated by scanning electron microscopy and electrochemical impedance spectroscopy (EIS). The EIS results showed that the addition of Ce(III) ions improved the corrosion resistance of the AA 2024-T3 TSA anodized and hydrothermally treated.