3 Dangers of chromium VI compounds The use of chromium VI has been restricted throughout the EU since 2013 and is only possible with the appropriate approval in defined uses until September 2024. Corresponding chromium VI-free alternatives are available and have proven themselves in practice. The uncontrolled formation of chromium VI, e.g., with HT screw pastes in combination with high-alloy steels, is a significant risk for maintenance staff, especially from a health point of view. Chromium VI has a strong carcinogenic (carcinogenic) and mutagenic effect. In addition, chromium VI is very toxic, 0.6 g taken orally has a fatal effect on humans. (Figure 3) Research 10 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0027 1 Emergence of the problem High temperature (HT) screw pastes have been used successfully in almost all areas of industry for decades. Without these pastes, the correct assembly of a high-alloy screw connection is not possible. And they then facilitate their non-destructive dismantling. Over the years, the pastes have evolved: from metal-containing pastes (copper, aluminum, or nickel alloy pastes) to metal-free pastes (“ceramic pastes”) towards high-purity pastes to exclude reactions with the screw material in critical applications and to improve the function of the connection so that it cannot be influenced even under extreme conditions. (Figure 1) Chromium-VI formation on high alloyed screws Rüdiger Schiffer* HT screw pastes, high-alloy screw connection, dangers, chromium VI-free alternatives, detection Keywords * Rüdiger Schiffer OKS Spezialschmierstoffe GmbH, Germany Figure 1 Figure 3 At the end of 2020/ beginning of 2021, chrome VI was detected when high-alloy screw connections in turbines were dismantled. Subsequent investigations found that calcium hydroxide, one of the essential components of modern HT screw pastes, is the cause. 2 Formation of chromium VI in screw connections In the case of steels with a high chromium content, the calcium hydroxide acts as a catalyst and temperatures above 300 °C then promote the formation of chromate, including chromium VI. (Figure 2 right) Chromium VI is also known as a contact allergen which has a skin-irritating effect (e.g., in the past when leather was made). So, there are good reasons to prevent the uncontrolled formation of chromium VI if possible. 4 Detection of chromium VI compounds The knowledge gained about the formation of chromium VI with certain material combinations and operating conditions naturally raised the question of how the formation of chromium IV in high-alloy, chromium-containing steels can be prevented in combination with the high-temperature pastes that are currently commercially available, since most of these HT screw pastes contain calcium hydroxide. To do this, however, a methodology first had to be developed to reliably simulate the formation of chromium VI without exposing the employees in the laboratory to its dangers. Standard stainless steel balls, such as those used in roller bearings, are completely immersed in the paste and stored in an oven for a certain period of time at a temperature well in excess of 300 °C. The remains of the paste then remaining on the surface of the balls were dissolved in water. This makes it relatively easy to detect and measure chromium VI. As soon as the residues are dissolved, you can see whether little or much chromium VI has been formed. Light yellow indicates little or no chromium VI, pink or violet rather a lot. The chromium VI content of the solution is then measured. (Figure 4) 5 Solution approach Theoretically relatively simple: A - if possible white or light-colored - HT screw paste only without calcium hydroxide with largely comparable technical properties in terms of temperature range, coefficients of friction and dismantling. In addition, free of disruptive substances such as Sulphur, chlorine, etc. and - perhaps also - suitable for use in food technology. (Table 1) In practice, it wasn’t that easy after all. The coefficients of friction on V2A and V4A were initially much too high. 6 First results In the meantime, the development of an HT screw paste that does not form chromium VI in combination with high chromium steels has been largely completed. At less than 0.05 ppm, the formation of chromium VI is below the detection limit of our measuring devices. The coefficients of friction on different surfaces lie reliably in the range of µ total = 0.14 with µ thread = 0.16 and µ head = 0.12 for V2A screws. The temperature range is more than 1,000 °C. The factor for the HT breakaway torque has not yet been determined, this will take place after the final release of the formulation. The light gray paste meets the requirements for NSF H1 registration. (Table 2) 7 Outlook A new generation of HT screw pastes comes onto the market. This new HT screw paste can really be used uni- Research 11 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0027 Figure 4 Criteria Standard Condition Unit Requirement Base Oil Type Synthetic Solid Lubricants Type Colour light colored Temperatur Range minimum service Temperature °C -30 maximum service Temperature Lubrication °C > 150 maximum service Temperature Separation °C > 900 Friction value DIN EN ISO 16 047 Screw ISO 4017 M10x55-8.8 black Nut ISO 4032 M10-10 black μ 0,12 DIN EN ISO 16 047 Screw ISO 4017 A2 M10x55-70 Nut ISO 4032 A2 M10-70 μ 0,12 HT- Break Loose Torque DIN 267-27 M10 A2, 40 Nm, 400°C, 100 h Nm < 2,5 x Tightening Torque Corrosion Protection Salt spray test ISO 9227 h, μm > 500, 50 Releases Food Industry NSF H1 Listings Customs world wide REACh conform SVHC not included RoHS conform Additional Requirements PFASfree not included Chrom-VIformation OKS Standard ppm < 0,05 Table 1 other technical requirements. In addition, this paste is suitable for industrial applications as well as for applications in food technology and comparable areas. Research 12 Tribologie + Schmierungstechnik · 70. Jahrgang · eOnly Sonderausgabe 1/ 2023 DOI 10.24053/ TuS-2023-0027 versally. The critical formation of chromium VI in highalloy steels in combination with extreme operating temperatures is reliably minimized without reducing the Criteria Standard Condition Unit Requirement New Paste Base Oil Type Synthetic Synthetic Solid Lubricants Type Solid lubricant mixture Colour light colored light grey/ grey Temperatur Range minimum service Temperature °C -30 not determined maximum service Temperature Lubrication °C > 150 not determined maximum service Temperature Separation °C > 900 > 1.000 Friction value DIN EN ISO 16 047 Screw ISO 4017 M10x55-8.8 black Nut ISO 4032 M10-10 black μ 0,12 not determined DIN EN ISO 16 047 Screw ISO 4017 A2 M10x55-70 Nut ISO 4032 A2 M10-70 μ 0,12 0,14 (μk 0,12, μg 0,16) HT- Break Loose Torque DIN 267-27 M10 A2, 40 Nm, 400°C, 100 h Nm < 2,5 x Tightening Torque not determined Corrosion Protection Salt spray test ISO 9227 h, μm > 500, 50 not determined Releases Food Industry NSF H1 H1 eligible Listings Customs world wide world wide REACh conform conform SVHC not included not included RoHS conform conform Additional Requirements PFASfree not included not included Chrom-VIformation OKS Standard ppm < 0,05 < 0,05 Table 2