ABSTRACT Type 2 aluminized steel has a layered aluminum/Al-Fe coating on a carbon steel substrate. It has been proposed that carbonate scales formed from natural waters are protective to this material but concern existed about stability of the passive layer under those conditions. The behavior of type 2 aluminized steel was studied for >3,000 h in waters of controlled varying scaling tendencies containing different amounts of Ca+2 and HCO3 - ions at room temperature and a moderate (370 ppm) chloride content. Initial findings showed extremely low corrosion rates under high total alkalinity and calcium carbonate precipitating tendencies. Corrosion performance was good over the test period in a high total alkalinity, nonscale forming medium but electrochemical impedance measurements indicated some corrosion in progress. Exposure to the same chloride content but in the absence of alkalinity or precipitating hardness led to early pitting of the aluminized layer.
INTRODUCTION Type 2 aluminized steel (T2AS) is produced as a steel sheet hot dip coated on both sides with commercially pure aluminum, which provides corrosion protection through low corrosion rate of the aluminum when it is passive, and by galvanic protection of exposed underlying steel under more aggressive conditions 1. T2AS is increasingly used for metallic drainage components in contact with natural waters. Corrosion is an important durability limitation factor in these components which are often designed for very long service life (e.g. 75 years) 2. Mechanistic knowledge of corrosion processes is needed to better forecasting durability in critical highway applications. In particular, it has been proposed 3,4 that calcium carbonate scales formed from natural waters are protective to aluminized steel.
A common indicator or scaling tendency is the Langelier Saturation Index, LSI = pHpHs, where pHs is the pH that would result in CaCO3 precipitation 5. LSI>0 implies a tendency for CaCO3 precipitation. However, the LSI does not consider the reserve of species in the solution responsible for a given pH. Based on extensive field data, Bednar 4 proposed that the corrosion performance of T2AS in aerated media may be better predicted by the combination of an index indicating carbonate scaling tendency (BI=Total Alkalinity (TA) plus Total Hardness (TH) minus Free CO2 (FC)) and the conductivity ó of the solution in contact with the alloy. However, there is concern that the aluminum-rich layer in T2AS could be susceptible to depassivation if the carbonate scale promotes alkaline conditions. For example, Porter and Hadden 6 stated that corrosion in pure aluminum was most severe in harder natural waters. This concern is addressed in the present work, where initial experiments to examine the room temperature behavior of T2AS in synthetic waters of controlled varying scaling tendencies are reported. Three environments were used corresponding to a carbonate precipitating condition (P), a mildly alkaline but non-precipitating condition (NP), and a neutral control. The conditions were obtained by adjusting the Ca+2 and HCO3 - ion contents.