11. From metallurgical roasting to high-temperature insulation

C. Insulation technology: hot components and insulation

The historical patent specification describes a deliberately intensified metallurgical roasting. Its significance for insulation materials on chromium-alloyed hot components lies in the reaction chemistry: the groups of substances relevant to calcium and sodium chromate are identical.

The German statutory accident insurance institution BG ETEM describes today's plant-related findings accordingly: contact between chromium-alloyed steels and insulation materials or assembly pastes containing alkali and alkaline earth metals can lead to the formation of carcinogenic chromates at steel temperatures of approximately 350 to 800 °C. As influencing factors, it names the chromium content of the steel, calcium, magnesium, sodium or potassium compounds, oxygen, air humidity and, from approximately 550 °C, also water vapour. According to BG ETEM, visible yellowish deposits on turbines and steam pressure pipes indicated calcium chromate and subsequently tested positive for chromium(VI) compounds on sampling. [2]

The application context was explicitly published before 2015

Toshiba's Japanese patent application JP2011232021A, published on 17 November 2011 with priority of 8 April 2010, describes precisely this interface: chromium oxide on stainless steel reacts with the calcium component of a thermal insulation under oxygen to form calcium chromate. The document explains the change from asbestos to calcium silicate and proposes a ceramic separating layer to suppress Cr(VI) formation. It also names potassium-, magnesium- and sodium-containing insulation. The application-related problem, including a prevention approach, was thus also publicly accessible before the 2015 study. [26]

As earlier literature, the patent cites Corrosion Center News No. 034 of 1 June 2005. This reference is a targeted archive trail; the original issue was not consulted here. The patent family identified is Japanese. Its public availability must be distinguished from the actual knowledge of individual European manufacturers. [26]

Examination of both Toshiba patents and of the reference "trade name Rockwool"

The reference "thermal insulation material (trade name: Rockwool)" is contained in both translations submitted. The corresponding product designation also appears in the Japanese original of JP2011232021A and in the Japanese granted version JP5774355B2 of the second application JP2012220174A. The reference was therefore not generated by the machine translation. Both test descriptions characterise the insulation as mainly calcium oxide/silicon oxide-based. [26, 38]

JP2011232021A tests SUS304 with and without ceramic separating layers at 600 °C for 500 hours in air. In the uncoated comparison, a calcium-chromium complex oxide and, in small quantities, a magnesium-chromium complex oxide are described at the contact surface by means of X-ray diffraction. The document explicitly explains the CaCrO₄ formation pathway with a reaction equation and also investigates sodium-containing insulation. The ceramic layer is intended to interrupt the contact of the reactive components. [26]

JP2012220174A, published on 12 November 2012 with priority of 14 April 2011, pursues a further prevention approach: a 200 µm thick pure nickel foil separates SUS304 from the insulation. After 600 °C and 500 hours in air, a yellow reaction product is described for direct contact without foil. This is analysed by XRD and designated by the authors as a Cr(VI) reaction product. The comparative tests with sodium-containing perlite and calcium silicate respectively also describe chromium-sodium and chromium-calcium complex oxides. For the set-up with foil, no corresponding visible reaction is reported. This comparison shows a targeted intervention in the interface reaction; the absence of a colour finding does not in itself imply a quantitative Cr(VI) value of zero. [38]

The wording "Rockwool" is documented; a manufacturer, a precise product grade or an attribution to ProRox is not identified in these test passages. In Japanese, the term is also used for stone wool. For a specific company attribution, product and supplier data would have to be obtained. The technical finding remains independent of this: Toshiba investigates calcium-containing insulation on chromium-containing metal and describes specific ways of interrupting the reaction. [26, 38]

Both documents explicitly name waste incineration plants, biomass firing systems, thermal power plants and chemical plants. The link to the combustion sector is thus already part of the original application description. The reaction between insulation and metal must be distinguished from Cr(VI) formation in the ash matrix; both can be relevant in the same plant. [26, 38]