AGA UPDATE
Question of the Month
Q: What is the formula for the silicon equivalent, and how can it be used to predict steel reactivity?
A: The silicon equivalent is a simple formula created by Dr. Sandelin that takes into consideration the
combined effect silicon and phosphorus have on the metallurgical reaction during hot-dip galvanizing.
Dr. Sandelin created the Sandelin Curve, which has become an invaluable tool used by galvanizers for
predicting the reactivity of steel to be galvanized, but the Sandelin Curve only accounts for silicon content
in steel. Dr. Sandelin realized phosphorus, the other catalyst in the galvanizing reaction, needed to be
accounted for in some way, hence the creation of the Silicon Equivalent formula:
Silicon Equivalent = Silicon content of the steel + 2.5*(Phosphorus content of the steel)
After the silicon and phosphorus levels have been inputted into the formula and the calculation has been
made, that value can then be plugged into the Sandelin Curve (Figure 1) to predict the reactivity of the steel.
The factor of 2.5 that is multiplied by the
phosphorus level can have a profound effect
on the reactivity of steels that have the same
silicon level. For example, take two steels
that have a silicon content of 0.17% but have
different values of phosphorus, say 0.01% and
0.08%. By plugging these values into the Silicon
Equivalent formula, we get:
Steel 1: 0.17% + 2.5*(0.01%) = 0.195%
Steel 2: 0.17% + 2.5*(0.08%) = 0.37%
So, although the two steels have similar silicon
levels, their reactivity is very different when the
phosphorus levels are considered. Looking at
Figure 1: Sandelin Curve
the Sandelin Curve shows that steel 1 would fall
into the ‘saddle’ area of the Sandelin curve and
could be considered non-reactive and would likely develop a normal galvanized coating. Steel 2, however,
would fall into the area after the ‘saddle’ area, where steel reactivity is high. Steel 2 would likely develop a
galvanized coating that is thicker and duller than non-reactive steel.
There are a few things that need to be considered when using the Silicon Equivalent or even the
Sandelin Curve to predict steel reactivity. Element levels can vary +/- 0.05% and the values listed on steel
certifications are only one sample taken from a heat/batch of steel. The element levels on the individual
pieces, and even on the same piece of steel, from the heat/batch can vary to some degree. This is to say,
predicting steel reactivity using the Sandelin Curve and Silicon Equivalent is not an exact science.
For additional information on steel reactivity, the Sandelin Curve, or the Silicon Equivalent, please contact
the AGA Technical Department at 720-554-0900 x21.