(a) Mill scale forms as a thin layer of iron oxide on the surface of steel plates or sheets during the manufacturing process. The causes of its formation are:

• Mill scale develops when red-hot iron or steel billets are rolled in rolling mills during the production process. The intense heat causes the surface of the metal to oxidize.
• Oxidation occurs periodically at elevated temperatures in dry conditions as the metal reacts with oxygen in the atmosphere, forming iron oxide layers on the surface.
• This thin oxide layer comprises a mix of iron oxides (e.g., FeO, Fe2O3, and Fe3O4), which adhere to the metal's surface during cooling.
• Mill scale aggravates corrosion by trapping moisture and acting as a barrier, preventing effective coating or painting unless removed during surface preparation.

(b) The preparation of a ship's underwater hull before applying a long-life coating in a dry dock involves a three-step process. This process addresses the removal of contaminants and the creation of a suitable surface profile.

(i) Washing: The hull surface must be thoroughly cleaned to remove all marine growth (algae, slime, etc.), accumulated salts, dirt, grease, and oil. High-pressure freshwater washing is the standard method for this initial cleaning. The goal is to present a clean substrate for subsequent stages.

(ii) Blasting: Abrasive blasting is the preferred method for removing rust, defective paint, and any remaining contaminants. This process achieves a bare metal surface, essential for proper adhesion of the new coating. The extent of blasting (localized or full hull) depends on the condition of the existing surface. The intensity and type of abrasive used are carefully controlled to achieve the desired surface roughness profile.

(iii) Primer Application: After blasting, the surface is again cleaned to remove any blasting debris. A primer coat is then applied to provide corrosion protection and to create an ideal surface for the subsequent topcoat adhesion. This primer acts as an intermediary layer, enhancing the bond between the substrate and the long-life coating system.

Wash Primer/Pretreatment Primer/Metal Conditioning Primer:

• These primers act as a base layer, improving adhesion of subsequent layers. Common types include epoxy primers pigmented with iron oxide and corrosion inhibiting pigments (zinc and calcium phosphates, although zinc content is minimized due to safety concerns).

Anticorrosive Coating:

• This layer primarily provides corrosion protection to the underlying metal. Two-component epoxies, coal tar epoxies, and epoxy or polyester coatings incorporating glass flakes are frequently employed. Glass flakes enhance mechanical strength and water vapor impermeability.

Antifouling Coating:

• This layer prevents the attachment of marine organisms (fouling). Historically, tin-based paints were used, but due to environmental regulations, they have been largely replaced by copper-based, silicone-based, or non-TBT (Tributyltin) self-polishing antifouling coatings. These newer coatings typically use seawater-soluble polymers. The number of antifouling layers applied (two or three) depends on the specific system chosen and required longevity.