2.6.2. Photo-Microscopy

The exposed sample surfaces were cleaned first in 15% HCl solution with the addition of 0.5% hexamethylenetetramine for 10 min, then rinsed in tap water, ultrasonically in alcohol for 2 min. and finally dried in blowing air. The topography of the exposed samples was analysed using optical microscopy to see if corrosion took place uniformly or locally at sample surfaces. The optical microscopes used were Olympus (DP200, for low magnification) and Leica (Reichert MEF 4 M, for high magnification) with the Infinity X camera and DeltaPix software. In the case of localized corrosion, pit depth was first measured manually using microscope Leica by turning the "Fine adjustment" from focusing on the sample surface to focusing on the bottom of the pits. The depth was calculated from

the turning scales on the "Fine adjustment" which was calibrated. The deepest pits were chosen to gain a cross-sectional view in order to measure the real depth to make sure no deposit at the pit bottom might hinder the light reaching the real bottom. Four deep pits on each sample were measured by cross-sectional view.

#### 2.6.3. Scanning Electron Microscopy (SEM)

The steel sample surface was first ground using grinding paper (SiC) till 2500 grit and then polished up to 1 μm. The microstructure such as inclusions and the compositions at the steel surface were analysed using SEM in combination with energy-dispersive spectroscopy (EDS). SEM was undertaken using a Jeol JSM 5800LV instrument equipped with a Noran instrument EDS system.
