六氟鋯酸添加對於AZ31B鎂合金上過錳酸鹽化成皮膜的影響

Abstract

鎂合金因為低密度與高比強度逐漸受到重視，尤其適合作為3C電子產品的機殼。然而，鎂合金因為其極高的化學活性，在一般的使用環境下即會發生腐蝕，因此需要化成皮膜的保護提升其抗蝕性。化成皮膜本身為金屬氧化物與鹽類，其存在會降低鎂合金的接觸阻抗，進而降低電磁遮蔽效應，對於其在電子產品上的應用相當不利。並且，鎂合金上常見的化成皮膜通常較厚，且具有脫水裂紋等缺陷，對於耐蝕性的增強有限。除此之外，AZ31B為單相鎂合金，然而因為熔煉以及合金元素的添加會使得α-鎂上有隨機分布的Al8Mn5析出物。這些介金屬相在化成過程中會產生伽凡尼腐蝕進而導致膜層不均。本研究即透過六氟鋯酸的添加進入過錳酸鹽的化成液中，藉由在底材上生成氟化鎂抑制強氧化劑過錳酸根帶來的劇烈腐蝕，從而沉積一個均勻且薄的膜層在AZ31B鎂合金上。 本研究首先以電子顯微鏡觀察其微觀結構，發現化成膜層可分為一層較為疏鬆的氧化鎂/氫氧化鎂和氟化鎂的內層，以及由氧化鎂/氫氧化鎂、氟化鎂和二氧化錳的所組成的緻密外層。過量的六氟鋯酸添加雖然會降低底材上膜層的厚度，但是將導致鋁錳界金屬相的去合金化，並且在原址之上覆蓋較厚的氧化層，導致接觸阻抗的增加。電化學分析證實六氟鋯酸的添加可以大幅降低鎂合金的腐蝕電流，並且提升交流阻抗測試中等效電路的總阻抗值。長時間浸泡測試中，含有六氟鋯酸的化成皮膜其脫色的情形較為緩和，說明此膜層在長時間的腐蝕環境中仍具有抵抗力。整體膜層厚度的下降也使得接觸阻抗的數值相較傳統的過錳酸鹽化成皮膜較低，其結果接近美國軍事規範MIL-DTL-5541F中的標準。

Magnesium and its alloys are gradually valued in recent years because of their low density, high specific strength and good electro-magnetic shielding effect. These extinguished properties make them potential candidates for casing of electronic devices. However, magnesium suffers from corrosion because of its extremely high chemical reactivity. Therefore, conversion coatings are required to protect magnesium substrates. Conversion films are usually oxides or metal salts which are poor electric conductors. Thick conversion coatings decrease the electrical conductivity of magnesium alloys, which undermines the electro-magnetic shielding effect of magnesium casing. Common conversion coatings on magnesium alloys are thick and full of dehydration cracks, providing limited improvement in corrosion resistance. Furthermore, the flatness of the conversion coating differs from the microstructure of the substrate. AZ31B is a single-phase alloy on which α-Mg is the dominant phase. However, Al8Mn5 intermetallic particles are randomly distributed on this alloy during solidification. Al8Mn5 precipitates are cathodic sites which inflicts galvanic corrosion, causing the uneven distribution of the film. In this research, a thin and uniform conversion coating is deposited on AZ31B magnesium alloy by adding H2ZrF6 into the permanganate solution. H2ZrF6 inhibits the massive dissolution of magnesium induced by permanganate ions which are strong oxidizing agents. This study starts from the observation of surface morphologies of the coatings using SEM and TEM. A double-layer structure is revealed: a loose inner layer composed of MgO/Mg(OH)2 and MgF2; a compact outer layer consisting of MgO/Mg(OH)2, MgF2 and MnO2. Excess H2ZrF6 reduces the thickness of the coatings, though, preferential dissolution of aluminum in Al8Mn5 particles is found. Also, severe galvanic corrosion and trench around Al-Mn sites induce a dome-like structure on the surface, resulting in the increase in electric contact resistance. Results of electro-chemical measurements show that the MgF2 containing films significantly decelerate the corrosion rate. Besides, EIS measurement indicates an increase in total impedance of the film. In the 24 hours immersion test, little portion of H2ZrF6 added conversion coating is damaged while specimen without H2ZrF6 is almost fully decolorized during the test. The electric contact resistance of H2ZrF6 added specimen is greatly lower than that without H2ZrF6. Its value is close to the standard set up in MIL-DTL-5541F.