For the formation of calcium chromate (CaCrO4), this much was now clear, the following are needed:
Calcium oxide - CaO
Chromium (III) oxide - Cr2O3
High temperatures (more than 250°C)
Oxygen - O2
Even if the dissertation of Dr. Francesca Paoletti brought the breakthrough in the research, it was necessary to deepen the investigations.
If you know what you are looking for, the results are not far.
There are countless scientific studies on the formation of chromates.
The most important ones are mentioned and linked here:
The following graph shows the influence of the calcium content on chromate formation:
The second graph shows the intensity with which alkali or alkaline earth metals react with the chromium (III) oxide to form alkali or alkaline earth chromates:
And this brings us back to Dr. Francesca Paoletti, who already in 2002 pointed out that the calcium experiments also apply to other alkali or alkaline earth metals, albeit with different intensities, as the above graph shows.
And thus it is clear which problem Babcock & Wilcox described in its warning message of that time (1993) (see blog: "all not so new..."):
"...insulation containing chromium, the chromium content of which could convert to hexavalent chromium compounds during the course of applications...".
The insulation materials used at those times contained, as we have seen
Chromium (III) oxide - Cr2O3
and, even it is hidden:
"...a series of oxides such as zirconia, iron oxide, titanium oxide, magnesium oxide, CALCIUMOXIDE, other alkaline earth oxides including sodium oxide, potassium oxide, barium oxides..."
used at high temperatures - I do not think it is necessary to go into further detail here about the oxygen present.
If we compare the above-mentioned oxides with the second diagram, it becomes clear that the mix of elements created an ideal basis for the formation of the hexavalent chromium compounds.
In the meantime, the chromium (III) oxide has disappeared from most insulating materials. If said Cr2O3 is the root of all evil, it must be removed, so far so good.
Is Cr2O3 really gone?
Unfortunately, the clear answer to the question is usually NO!
Even though chromium (III) oxide is no longer a direct component of insulating materials nowadays, we repeatedly encounter chromium (III) compounds in the high-temperature range.
Many insulating fabrics are stainless steel reinforced to give the fabrics the necessary strength.
Insulating mats are "quilted" with stainless steel wire or have a stainless steel grid structure, and most of these stainless steels contain chromium.
In particular, the alloy of the stainless steel contains chromium (III) oxide.
Even if stainless steel reinforcement or stiffening were to be dispensed with, contact between calcium oxide and chromium (III) oxide would unfortunately not be prevented as a rule!
why is this so?
Most bodies, be they turbines, machines, engines, boilers or pipelines, are nowadays made of stainless steels with a high chromium content and thus these alloys, unless they have been specially treated, you guessed it, contain chromium (III) oxide and thus, unfortunately, the so-called
squaring of the circles
and no matter how you turn it, in almost all areas of today's high-temperature insulation, calcium oxide-containing insulating materials come into contact with chromium (III)-oxide-containing stainless steels, and this at temperatures in which the formation of alkali or alkaline earth chromates only goes in one direction, and that is upwards!
The temperature range at which the chromates would decompose again, i.e. be "oxidized down" (from 850°C; the harmless chromium (III) oxide would then be formed again), is unfortunately outside the operating temperature of the above-mentioned plant temperatures.
Once formed, the carcinogenic, partly mutagenic and possibly also fertility-damaging and for the most part acutely environmentally harmful (chronically water-polluting with long-term consequences for the environment) chromates are thus released, at the latest when the insulation systems are dismantled, and endanger people and the environment, because this problem is either played down or ignored, or is not known to most users or has not been made known!