Atsushi Andoh

Improvement in the Oxidation Resistance of an Al-Deposited Fe–Cr–Al Foil by Preoxidation

The oxidation kinetics of conventional Fe–20Cr–5Al (in mass %) foil, Al-deposited foil and Al-deposited and preoxidized foil was studied at 1373 K in air. All the foils were 50-μm thick and contained minor additions of rare-earth elements. The oxide scales were observed with SEM and TEM combined with EDS and were characterized with X-ray diffractometry and electron diffraction. The deposition of Al onto the foil from the vapor phase improves oxidation resistance. The details regarding this matter were reported elsewhere. The combination of the Al deposition and the subsequent preoxidation at 1173 K for 90 ks in air further increases the oxidation resistance, i.e., the smallest parabolic rate constant among the three kinds of foils, and excellent scale adherence. Preoxidation enhances the growth of θ-Al2O3, which transforms to α-Al2O3 during subsequent oxidation. However, such α-Al2O3 grains are much larger than those formed on the conventional foil of similar chemical composition. Small closed voids and small spinel-type, oxide particles appear in α-Al2O3 grains with the progress of oxidation. The former is explained in terms of the volume decrease accompanying the phase transformation and the latter by the low solubility of Fe in α-Al2O3.

High-temperature oxidation of Al-deposited stainless-steel foils

The oxidation resistance of Al-deposited Fe−Cr−Al foils containing small amounts of La and Ce was assessed by a cyclic oxidation test with temperature varying between room temperature and 1323 K to 1423 K in static air. (1) The Al content of Fe−Cr−Al−La, Ce foils can be increased by depositing an Al layer from the vapor phase. The deposition of a 1-μm-thick Al layer on both sides of the 50-μm-thick foil is equivalent to a 1.5 mass% increase in the Al content. The deposited Al diffuses into the foil during heat treatment. The uniform distribution of Al is obtained by heating at 1273 K for 18 ks. (2) After the initial transition stage the oxidation follows the parabolic law until breakaway sets in. The scale consists mainly of α-Al2O3 during the parabolic period. (3) The increase in the Al content by more than 5 mass% by the Al-deposition remarkably improves high-temperature oxidation resistance (smaller parabolic rate constant and longer protection time). (4) The Al-deposited foils have better oxidation resistance than the conventional foils with the same contents of Al and rare-earth elements. This is attributable to the different nature of the initially formed oxide on the Al-deposited foil. (5) The so-called rare-earth element effect was also observed for the Al-deposited foils. Predominant diffusion of oxygen through the Al2O3 scale and vacancy-sink mechanism are applicable to the present results.

Application of Hot-Dip Zn-6%Al-3%Mg Alloy Coated Steel Sheet to Automotive Body

当社は2000年に溶融Zn-6mass%Al-3mass%Mg合金 めっき鋼板(以下,ZAMと記す)の営業生産を開始し, プレハブ住宅の構造材をはじめ建築資材,道路資材,農 業資材,家電といった種々の用途への展開を図ってきた。 最近はZAMの高耐食性を活かし,自動車部品分野での 採用量も増加しており,将来的には自動車ボディ材への 展開も期待される。 自動車ボディ材料に関しては,1980年代に北米や北 欧で融雪剤散布による穴あき腐食が問題視されると,そ の解決策の一つとして亜鉛めっき鋼板の使用が急速に広 まった。さらに,自動車ボディの腐食に関する保証年 数が引き伸ばされるにともない,亜鉛めっきの付着量を 高める対策がとられてきた。その一方,とくに最近の 欧州において自動車ボディの組み立てにレーザー溶接が 積極的に取り入れられるようになり,溶接性の観点か ら亜鉛めっき付着量の低減が求められるようになってきて いる。それらを解決する方策として,低付着量でも高耐食 性が得られる亜鉛-マグネシウム合金めっき鋼板の開発 溶融Zn-6%Al-3%Mg合金めっき鋼板の自動車ボディへの適用