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Atmospheric Corrosion
Published in Mavis Sika Okyere, Corrosion Protection for the Oil and Gas Industry, 2019
Overall, there are three categories of conversion coating: 1. Oxide conversion coating: This type of coating is an anticorrosion product that is ultrathin and offers good adhesion. In such cases, oxide treatments can be performed through electrochemical, heat, or chemical reactions. The best examples of oxide coatings include chemical baths, black oxide, and anodizing.2. Phosphate conversion coating: This is produced by the chemical conversion that exists on a metal substrate in order to produce a highly adhesive phosphate coating. The crystals that may build up on the surface include manganese, zinc, and iron. Out of these three, phosphate of manganese is the best type of coating for wear applications. It is ideal for low alloy metals, cast iron, and carbon steel. It is one of the most beneficial forms of coating material that is nonmetallic.3. Chromate conversion coating: This is comparable to phosphate coating since it is created by chemical conversion. It is formed through the interaction of chromium salts or chromium acid with water solutions. This type of coating is applicable to zinc, aluminum, magnesium, and cadmium. This coating normally provides superior corrosive resistance and is broadly used in giving protection to usual household products such as hinges, screws, and other hardware items.
Synthesis and degradation behaviour of Zn-modified coating on Mg alloy
Published in Surface Engineering, 2021
Jun Wang, Chaoyang Jin, Di Mei, Yan Ding, Lei Chang, Shijie Zhu, Liguo Wang, Yashan Feng, Shaokang Guan
The chemical compositions of the areas J–O in Figure 1(a,d,g,h) analysed via EDS are shown in Table 1. On comparing between areas J and K, Mg is detected in the new small crystals (area K) but not in the grown crystals (area J). That is, Mg2+, Ca2+ and Zn2+ participate in crystal nucleation; only Ca2+ and Zn2+ participate in crystal growth. Furthermore, only a small amount of Mg2+ is produced during electrodeposition, indicating that corrosion of substrates is suppressed during dual-pulse electrodeposition. In the contrast, during phosphation, substrates are continuously corroded by phosphating solution, thus deteriorating the morphology of the Zn–Ca phosphate conversion coating on Mg substrate [25]. Moreover, elemental Mg was detected in undamaged coating (area M). Our results show that the coating generated for 50 min has a certain thickness and the molar ratios of Zn/Ca and (Zn + Ca)/P are approximately 2.08 and 1.42, respectively. Since N is the area around the pores and elemental Mg is again detected in this region indicating that the pores can damage the coating and expose the substrate, and the protection capacity of the coating for the substrate correspondingly weakens.
EUROCORR 2018: applied science with constant awareness
Published in Corrosion Engineering, Science and Technology, 2019
A poster entitled ‘Effect of pre-treatment on corrosion resistance of phosphate conversion coating on Mg alloy’ was given by Yuyang Chen (Harbin Engineering University, China). Specimens of Mg-Gd-Y-Zr (Mg-RE) were immersed in a coupling agent and dipped into SiO2-containing sol–gel to produce a Pre-PCC coating. The coatings were investigated by various techniques including hydrogen evolution and PP in 3.5 wt-% NaCl saturated with Mg(OH)2. The volume of H2 evolved after 48 h decreased from 21.3 to –2.7 ml cm−2 for PCC, 4.5 ml cm−2 for CCC. PP revealed a substantially increased corrosion potential of the Mg alloy (from −1.63 to −1.43 V) after conversion coating, although a lower corrosion current density was indicated (PCC = 2.43 × 10−5 A cm−2; CCC = 2.11 × 10−3 A cm−2) relative to the bare alloy (9.28 × 10−5 A cm−2). The discrepancy was ascribed to the short-term nature of the polarisation test.
EUROCORR 2017 in combination with the 20th International Corrosion Congress and the Process Safety Congress 2017: Corrosion Control for Safer Living. Part 2
Published in Corrosion Engineering, Science and Technology, 2018
A poster describing ‘Phosphate conversion coating on Zn-based multiphase substrate: impact of activator’ was presented by Perrine Tanguy (Institut de Recherche de Chimie Paris, France). ZnAlMg multiphase sacrificial coatings on steel are regarded as a replacement of conventional HDG steel in many applications including automotive and pre-painted market. This work focused on the distribution of the phosphate crystals at initial steps of the Zn–Ni PCC formation on ZnAlMg multiphase surface and on the impact of a Ti-based activator on the morphology and formation kinetics of the layer. With or without activator, the phosphate crystals were not preferentially formed on specific phases but were observed on all phases. In the absence of the activator phosphate, crystal growth was faster than nucleation, leading to an inhomogeneous coating composed of isolated large aggregates. The presence of activator significantly accelerated the nucleation process, ensuring a compact layer of phosphate crystals 1–2 μm in diameter.