Corrosion in steel and the role of shot blasting

Corrosion is an electrochemical reaction: the iron in steel loses electrons when it comes into contact with moisture and oxygen, forming iron oxides — the visible rust — that do not protect the surface but progressively consume it.
Mill scale (the rolling oxide layer that hot-rolled steel carries from the manufacturing process) is one of the most common starting points. Although hard and well bonded, it has a different electrical potential from the base steel: when water enters, a localized galvanic cell forms that corrodes the steel beneath the mill scale itself. From the outside, the surface may appear intact; underneath, corrosion has already advanced.
Factors that accelerate the process include: high relative humidity, salinity (coastal or industrial environments), temperature, acidic or aggressive gases, and the presence of existing rust pitting. The most exposed industrial sectors — energy, shipbuilding, road infrastructure, agribusiness — face this problem on a permanent basis.

Painting directly over a surface with mill scale, rust or grease is the most frequent cause of premature coating failure. The mechanism is simple: mill scale has a different thermal expansion coefficient from steel. With temperature changes it cracks; water enters through the crack; corrosion advances beneath the paint without being visible from outside, until the coating blisters and peels off.
Without adequate anchor roughness on the surface, paint does not adhere mechanically either: the film is held only by surface tension, and any impact or stress lifts it. International standards (ISO 8501-1, SSPC/AMPP) define the required cleanliness grades (Sa 2, Sa 2½, Sa 3) according to coating type and environmental exposure. The detail of each grade is covered in our surface preparation standards report.

Shot blasting acts on two fronts simultaneously:
• Cleaning: projects abrasive at high speed and removes mill scale, rust, grease and any other contaminant down to bare steel. It is the only process that can remove mill scale from hot-rolled steel mechanically, without acids.
• Surface profile: the abrasive leaves a controlled roughness profile on the surface (Ra and Rz to specification). This roughness increases the contact area between the metal and the coating, generating the mechanical anchor the paint needs to last.

The result: a structure treated to Sa 2½ with the correct roughness profile and an appropriate paint system can multiply its service life by 3 to 5 times compared with the same structure painted without prior preparation.