Flame cleaning, also known as flame gouging, is the process of cleaning a structural steel surface by passing an intensely hot acetylene-oxygen flame over it. Mill scale

Mill scale

Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, iron oxides consisting of Iron oxide, hematite, and magnetite....

 and rust

Rust

Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...

 are removed by the reducing effect of the flame and the action of the heat, leaving the surface in a condition suitable for wire brushing and painting.

Overview

There are many uses for flame cleaning rather than just to remove rust and mill scale. It is also used as a removal tool for paints, bad welds, burrs, mechanical wear (scraping, cuts, gouges and more) along with other surface imperfections.
The process of flame cleaning is not anything new and does not require any extra equipment than what would be found in a typical metal shop. It is just an oxygen-fuel torch

Torch

A torch is a fire source, usually a rod-shaped piece of wood with a rag soaked in pitch and/or some other flammable material wrapped around one end. Torches were often supported in sconces by brackets high up on walls, to throw light over corridors in stone structures such as castles or crypts...

that is used parallel with the surface to melt and blow off any blemishes that the operator wants to eliminate. Shown in the diagram to the right, the surface is blasted with the oxygen-fuel torch across the surface versus the orthodox use of the flame as a cutting or gouging tool. Though the equipment is the same, the flame used in cleaning is drastically reduced to prevent excess removal of the surface. Various kinds of fuel can be run with the oxygen, for example acetylene.

Various surfaces can be cleaned using this process. The oxy-flame can work on assorted surface qualities and remove some of the hardest to clean substances such as lubricants and grease. Ceramics, steel constructions, offshore systems and even concrete elements can all be cleaned. Although the function of cleaning can be thoroughly accomplished, the general aesthetic look is damaged due to the burn marks by the oxy-flame. The surface is then ready for further processing such as painting, grinding or any other further operation.