The corrosion behavior of chromated AA2024 aluminum alloy in 3.5% NaCl solution

Purpose A chromate conversion coating was prepared on the surface of bare AA2024 aluminum alloy by direct immersion in the chromating treatment bath, and the corrosion behavior of chromated AA2024 aluminum alloy in 3.5 per cent NaCl solution was studied by electrochemical measurement and microstructural observation. Design/methodology/approach According to the polarization curve test and the scanning electron microscope observation, the corrosion evolution of chromated AA2024 in 3.5 per cent NaCl solution was divided into the following three stages: coating failure, pitting corrosion and intergranular corrosion (IGC). Findings In the first stage, the chromate coating degraded gradually due to the combined action of chloride anions and water molecules, resulting in the complete exposure of AA2024 substrate to 3.5 per cent NaCl solution. Subsequently, in the second stage, chloride anions adsorbed at the sites of θ phase (Al2Cu) and S phase (Al2CuMg) on the AA2024 surface preferentially, and some corrosion pits initiated at the above two sites and propagated towards the deep of crystal grains. However, the propagation of a pit terminated when the pit front arrived at the adjacent grain boundary, where the initiation of IGC occurred. Originality/value Finally, in the third stage, the corrosion proceeded along the continuous grain boundary net and penetrated the internal of AA2024 substrate, resulting in the propagation of IGC. The related corrosion mechanisms for the bare and the chromated AA2024 were also discussed.

ANALISA PENGARUH JARAK KATODA DAN ANODA DALAM PROSES ELEKTROPLATING ALUMINIUM TERHADAP KETEBALAN LAPISAN

Aluminium banyak digunakan baik pada peralatan rumah tangga maupun pada industri. Tujuan dari penelitian ini adalah untuk mendapatkan gambaran yang nyata pengaruh jarak anoda dan katoda pelapisan nikel pada proses chromate coating logam aluminium terhadap daya ikatan lapisan danketebalan lapisan. Spesimen yang dilapisiadalah logam aluminium berbentuk lingkaran diameter 50 mmdantebal 3 mm. Spesimen dilapisi nikeldengan proses electroplating dalam larutannikel dan larutan krom denganvariasijarak katoda 10, 15, 20 cm. Proses elektroplating nikel dilakukan dengan menggunakan larutan elektrolit Watts, dengan arus 5 ampere dan tegangan 2 volt dengan waktu 30 menit, sedangkan pada elektroplating krom dilakukan dengan arus 30 ampere, dan tegangan 12 volt selama 4 detik. Data yang diperoleh secara eksperimen, kemudian dilakukan pengukuran ketebalan untuk melihat pengaruh jarak katoda dan anoda terhadap ketebalan lapisan.Hasil penelitian menunjukkan bahwa ada pengaruh jarak anoda terhadap tampilan dekoratif permukaan dan ketebalan lapisan. Hasil terbaik diperoleh pada jarak anoda dan katoda 15 cm, yaitu ketebalan lapisan 11,712 µm dan tampilan spesimen lapisan lebih rata dan lebih mengkilat. Kata Kunci: elektroplating, aluminium, ketebalan lapisan

Anticorrosion Nanocrystalline Beta Zeolite Thin Film for Advanced Applications

Steel alloys corrosion is ubiquitous and is conventionally protected by anticorrosion chromate coatings. However, the process suffers from the release of carcinogenic hexavalent chromium ions that needs to be replaced by an ecofriendly alternative. In this context, the need for the development of satisfactory ecofriendly chromium-free coating with superior corrosion performance is highly desirable. In the present study, we synthesized fully dispersible nanocrystalline Beta zeolite seeds and coated on steel alloys followed by steaming. The samples were characterized by XRD, FE-SEM, and DLS analyses. The anticorrosion behavior of the synthesized nanoparticle coatings on steel alloys was investigated by electrochemical measurements (DC polarization) and electrochemical impedance spectroscopy (EIS) in NaCl and acid and alkaline media under identical experimental conditions. The present study demonstrated that the nanozeolite coating can be a potential alternative for toxic and carcinogenic chromate coating.

Corrosion Behaviors of Thermal Diffusion Coating on the Surface of Inconel625 Alloy in Chloride Molten Salts

In solar thermal storage system, the mixed chloride molten salt with the higher conversion efficiency than a single molten salt, but they are more corrosive than the often used nitride molten salts. In the presents work, aluminide and chromate coating were prepared on the surface of Inconel625 alloy by thermal packing method. The corrosion behaviors of thermal diffusion coating on the surface of Inconel625 alloy in mixed molten salts at 900°C were studied by using XRD and SEM equipped with EDS in the present work. The results showed that both of the two thermal diffusion coatings have sever corrosion in the mixed chloride molten salts, but thermal diffusion Al coating in the mixed chloride molten salt corrosion is more serious than thermal diffusion Cr coating, because Cr2O3is more easily dissolved in the molten salt than Al2O3.

A replacement of chromate coating for adhesive bonding of stainless steel in a corrosive environment

For adhesive bonding of stainless steel to itself, a surface treatment involving chromates is used. However, chromates are environmentally unfriendly so a replacement is being sought. In this paper, an alternative to chromate was investigated. The standard test method, ASTM D 1002, was used to measure the failure load of adhesively bonded stainless steel samples. A general-purpose epoxy adhesive was used. To simulate marine exposure, adhesively bonded samples were placed in a 5% salt spray for extended periods of time, up to five weeks. Results indicated that the initial shear strength of adhesive joints prepared with a traditional chromate preparation was 25% greater than the new, alternative coating. However, more importantly, the rate of decrease in strength with salt spray exposure was greater for the chromate than for the alternative. After 21 days, both bonding surface treatments had the same strength. However, after this period of time, the alternative was stronger than the chromate treatment, indicating that the alternative was a more durable coating. An adhesive/adherent coated system was investigated using a finite element method in order to investigate the influence of adhesive thickness between the adhesive and the adherent, and the residual stress in the adhesive layer. Keywords: Adhesive, Alternative Coating, Crevice Corrosion, Potentio-Dynamic, Salt Spray.

Study of Corrosion Behaviour of Molybdenum Series Coatings on Galvanized Steel

In this paper, an organic/inorganic molybdenum series Cr-free coating was formed on galvanized steel by simple immersion and its corrosion behavior was compared to that of a typical chromate coating. Molybdate and 1-Hydroxy-ethylidene-1, 1-diphosphonic acid (HEDP) were used as corrosion inhibitor, as well as acrylic resin and silane were used as film-former and coupling agents, respectively. The corrosion behavior of the coatings was evaluated by Neutral salt spray (NSS), Electrochemical impedance spectroscopy (EIS) and Tafel polarization. The surface topography of the samples was observed by Scanning Electron Microscopy (SEM). The results indicated that the corroded area of the Mo-HEDP treatment was only corroded 2% after 72 h spraying, while the corrosion behaviour of Mo-HEDP was closed to that of Cr pretreatment due to the synergistic reaction of molybdate and HEDP. Compared with the film of Cr treatment, Mo-HEDP passivating coating was more environmentally friendly.