On hot rolled bars to be cold drawn, the dark oxide surface is called scale.
What we see on the surface is "scale"- a combination of oxides of iron.
Scale is the name given to the oxides of iron that are formed on as wrought products as a result of mill operations (high temperature rolling or furnace treatment)
Rust is the commonly used term for iron oxide from weathering or corrosion.
Scale is
Hard
Brittle
High Coefficient of Friction
So we need to get it off the steel if you are to have any chance of keeping the tool edge sharp.
There are 3 oxides of iron:
Hematite ( Fe2 O3) has a microhardness of ~ 1030 D.P.H., is red in color, and is not soluble in acid.
Magnetite (Fe3O4) has a microhardness of ~ 420-500 D.P.H., is black in color, and is not soluble in acid.
Wustite (FeO) has a microhardness of ~ 270 -350 D.P.H., is blueish in color, and is soluble in acid. Wustite is the phase that makes up the innermost scale on the bars or rods.
Hematite and Magnetite make up the outer layers of the scale, and due to their composition, make up the larger mass of scale present. Due to their hardness and quantity they are the real dealbreakers for machining as they create tool edge wear.
One of the ways that Cold Finished Steel bars aid machinability is by removing these hard abrasive oxides from the workpiece, so that they don’t destroy your tools and contaminate your cutting fluids.
Yes, you put on rust preventive. Yet, some parts seem more prone to rust than others.
Why do some parts rust more easily than others?
PMPA members rely on the members only Listserves to collectively solve problems and to advance everyone’s knowledge of the craft.
So it was no surprise to me at the end of July to see a post on PMPA’s Quality Listserve asking if some steels are more prone to rusting than others.
That same morning I emptied 45 pints of water out of the basement dehumidifier…
I counted six member company respondents to the original question, including one respondent with a specific example. Plus PMPA staff contributed a reply. This is the value of PMPA Member only Listserves. Every respondent is in the trade, and their experience and expertise is known- no Aliases.
So what are some of the factors that can contribute to your regular carbon or alloy steel parts rusting- Here is a brief summary of the factors brought up:
Steel related factors
The steels’ electrode potential
Chemical composition- unless a self-healing film is generated by the alloying additions, more complicated chemistry means more likely to rust;
Physically non-homogenous surface – texture, pits and cracks and crevices
Processing related factors
Is a film existing from prior operations (scale from mill rolling, oil from machining etc.) to prevent permeation of Oxygen and water-and is it intact?
Is there a significant difference in section thickness?
Is the part or places on the part highly stressed? (Stress can help accelerate the reaction)
Were other substances deposited on the material which could accelerate or inhibit rusting (Metal fines, salts, carbon ‘smut’ , acids or bases)?
Does a result of the process leave cracks crevices, or other similar features?
Environmental factors
Solution in contact (presence and type)
Hydrogen ion availability (pH) of the moisture
Oxygen level
Other ions species, their type and electroactivity
Flow or exchange rate of solution (or stagnation)
Degree of wetting
Temperature / Humidity time of year
Location: inside, controlled climate, or in transit
Is their cyclic stress?
Finally in the case where two or more dissimilar metals are in contact ( Even welded) the electrochemical potentials of these being different will result in the most anodic (Active) (A= anode = active) will be corroded by the most noble (Noble= Not as reactive).
Of course, all of these factors can interact, and I note that the rust question is a little early this year, it usually arrived about mid- August most years when I was handling claims at the steel company.
This is the meat of the PMPA Listserve discussion. What we didn’t provide here in this post were the specific tips, tricks, product recommendations and other insights that some poster’s shared regarding ther packaging and rust preventive application to prevent Rust from occurring in various situations.
PMPA Listserves- Advancing member competitiveness with each question asked.
