The Effect of Ammonium Ions on the Corrosion Resistance of Zinc - Aluminium and Zinc - Aluminium - Magnesium Coated Steel

Abstract

Galvanizing is the process of applying a protective zinc or zinc alloy coating to steel or iron which acts as a sacrificial anode to protect the underlying steel or iron. These coatings are mainly used in the automotive and construction industry where corrosive environments will be encountered. There has been a drive to improve the performance of galvanised coatings through alloy additions, initially Al (5 – 55 wt%) and more recently through the development of Zn - Al - Mg coatings. Here, Al and Mg are added as minority additions (typically 1 - 3 wt %) to improve corrosion resistance allowing thinner coatings to be used thus saving material, weight and cost. The assessment of the corrosion performance of such coatings is often evaluated through standardised tests. An industry standard test known as Prohesion or ASTM G85 Annex A5 is carried out on metallic coatings as it is thought to better represent long-term natural exposure than a regular salt spray test. The Prohesion test is a cyclic accelerated corrosion test using an electrolyte of 0.4% Ammonia sulphate and 0.05% sodium chloride (NaCl).This investigation aims to determine the effects on the corrosion performance of zinc-aluminium alloys and zinc-aluminium-magnesium alloys of varying compositions immersed in Prohesion solution. Previous work on this topic suggests that magnesium containing alloys were particularly susceptible to accelerated corrosion in Prohesion solution. Here, several zinc based alloys were immersed in Prohesion solution as well as a control 0.45% sodium chloride solution and subjected to electrochemical testing. The electrochemical techniques used to test said substrates are linear polarisation resistance (LPR), scanning vibrating electrode technique (SVET) and time-lapse optical microscopy (TLM) all for 24 hours. The alloys that underwent testing are shown in Table 1. Table 1: Table showing the composition of substrates used.Sample number Composition 1 Zn-<0.2wt%Al 2 Zn-5wt%Al 3 Zn-55wt%Al 4 Zn-1.2wt%Al-Zn1.2wt%Mg 5 Zn-3.5wt%Al-3wt%Mg 6 Zn-6wt%Al-3wt%Mg The results from the TLM showed differences in the corrosion mechanism that is dependent on the alloy composition and electrolyte. Generally, all substrates immersed in the control 0.45% NaCl showed localised corrosion with distinct anodes developing surrounded by a ring of corrosion product and white rust is observed on the surface of the substrate. The initial site of anodic attack in the microstructure was dependent on the alloy composition. However, when immersed in the Prohesion solution significant differences were observed dependent on the alloy composition. For Zn-Al-Mg coatings rapid attack of zinc-magnesium phases was observed especially in the eutectic regions followed by aggressive attack of the entire exposed surface of the alloy within 6 hours. Additionally, no corrosion product rings formed. For Zn-Al alloy coatings, the zinc phases were preferentially attacked with the aluminium containing phases appearing to be resistant to corrosion during the duration of the tests. No white rust was seen on the surface of the substrates and instead a dark surface/corrosion product film was observed. The corrosion rate from the LPR data measured at hourly intervals over 24 hours increases when substrates are immersed in Prohesion solution compared to NaCl, with the most noticeable increase seen when there are zinc-magnesium phases present within the alloy. The largest increase in corrosion rate mm per year (mmpy) was seen in sample 5 with it increasing from 0.15 mmpy after 24 hours in the control solution to 0.55 mmpy in the Prohesion solution after 24 hours. The same is observed with the SVET results showing that the ammonium ions are having a significant damaging effect on the Zn-Al-Mg metallic coatings. The proposed corrosion mechanism for Zn-Al-Mg alloy coatings in Prohesion solution is preferential dissolution of MgZn2 in eutectic phases due to reactivity of Mg producing Mg(OH)2 which is a highly basic hydroxide. Due to the basic nature of Mg(OH)2, Ammonium (NH4 +) cations react with Mg(OH)2 producing Mg2+ and Ammonia (NH3).Mg(OH)2+2NH4 +=Mg2++2NH3+2H2OZn(OH)2 does not undergo this reaction to the same extent as it hydrolyses in water (producing H+) therefore is not basic enough to support much NH3 production. Neutral NH3 reacts with metallic Zn or Zn2+ compounds to produce highly soluble zinc (II) tetra-ammine complex cations. Therefore, the protective Zn and Mg compounds forming at the metal surface will dissolve and the surface will become activated. This is demonstrated by the lack of white rust shown from samples immersed in the Prohesion solution. Al phases are resistant to NH3 hence the better performance of Zn-Al coatings and the preferential attack of the Zn in the Zn-Al timelapse microscopy.