TPT September 2022

AR T I C L E

Quaker Houghton

Tube corrosi n mechanisms and processes: understanding the basic elements that cause rust is the first step in preventing it from forming By Susan Conley, product application manager – tube and pipe for the Americas, Quaker Houghton

Shandong Province Sifang Technical Development Group

Whether the corrosion takes the form of red rust (ferric oxide, Fe 2 O 3 ) or black stain (ferrous oxide, Fe 3 O 4 ), the process is similar: oxidation of the metal is linked to reduction of other constituents in the process, including the metalworking fluids. Defining corrosion Corrosion is a chemical process. Specifically for ferrous metal corrosion, it is the oxidation of iron metal from Fe to Fe+2, further to Fe+3, caused by electrons flowing from an anode (a point of negative polarity) to a cathode (a point of positive polarity). A common battery uses chemicals to carry electrical current from one terminal to the other; this is the same process that creates rust. Corrosion control processes stop the flow of electrons or disrupt the reaction at the cathode or anode. Rust requirements Three components or constituents are necessary for rust to form: • A cell, consisting of a cathode and an anode • Moisture, which provides a pathway for current flow • Oxygen, which combines with the metal Moisture and oxygen are straightforward, but the cell can be a bit mysterious. Six common conditions can turn any piece of steel into a corrosion cell (see Figure 1):

Applying a corrosion prevention product to steel is not enough. If the work piece is contaminated with something as minor as fingerprints or steel fines, corrosion can get a foothold. Understanding the components that make up a corrosion cell, and how a corrosion cell works, is necessary in learning how to prevent corrosion from getting a start. As a metal fabricator, most of your focus is on changing the shape of a metal workpiece, whether you are cutting, bending, end forming, piercing, notching, machining, or some other process. Most of these require an oil-, solvent-, or water-based fluid to prevent friction, which prevents overheating or premature wear. Another consideration, one no less important than fabricating the workpiece, is preventing corrosion. Some fabricators rely on the metalworking fluid to provide both in-process and final corrosion protection; others use a final process to apply a short- to long-term corrosion preventive. Either way, corrosion prevention agents provide a necessary function. Without protection, the iron (Fe) in the steel interacts with oxygen (O) in the atmosphere, and the steel begins to corrode.

Cell

Anode Cathode

Steel with existing rust or scale

Steel Steel

Rust or scale Dust or fines

Steel with dust or fines

Steel with electrolyte on surface of varying concentration

Steel

Acid, salt or alkali

Steel with fingerprints

Steel

Residue from oily skin

Stained steel

Annealed steel (coarse crystals)

Unevenly annealed steel

Steel with deep scratch (deep enough to alter surface oxides)

Scratched steel

Unscratched steel

Figure 1: Rust is caused by corrosion cells. Every corrosion cell has an anode and a cathode (positive and negative pole). Moisture provides a pathway for current flow and oxygen is the agent that causes steel to change form to ferric or ferrous oxide

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SEPTEMBER 2022