EP0933443B1 - Use of steel powder based on Fe-Cr-Si for corrosion resistant coatings - Google Patents

Abstract

High chromium and silicon content steel powder, for thermal coating of metal parts exposed to chlorine, chloride and/or sulfate corrosion. A powder of an alloy steel of composition (by wt.) more than 20 to 50% Cr, more than 3 to 10% Si, balance Fe and impurities is used for thermal coating of metal parts which are exposed, during use, to corrosive attack by chlorine-, chloride- and/or sulfate-containing media.

Description

The invention relates to coatings made of ferritic Steel alloys on metal components, primarily low alloy steel, preferably pipes and Pipe walls, for the purpose of corrosion protection against hot media containing chlorine and / or chloride and / or sulfate e.g. for plant components for thermal Waste disposal or copper refining.

In plants for energy and material conversion is at the There is often scope for the choice of process parameters which allows the process to be run in such a way that the Requirements for the heat resistance of the necessary components to be used by low-alloy steels can be met. So will e.g. in power plants that use electrical Generate electricity via a steam turbine, the superheated steam state choose so that the material-consuming Heat exchangers are made of low-alloy steel can. The efficiency improvement in the transition to higher temperatures and pressures justify it Additional costs for high-alloy, austenitic materials in usually not.

In plants for thermal waste disposal, the Service life of the heat exchangers is not, however, due to the thermal and mechanical properties but rather through the corrosion resistance of the Pipe material determined, which is therefore important Criterion for the choice of materials and the Litigation must be considered.

The in such plants depending on flue gas and Set pipe wall temperature occurring corrosion processes itself in a complicated way from the chlorine-catalyzed active oxidation in the presence of chlorine Gases or chlorides occur in oxidizing atmospheres and mainly the iron from the pipe material consumed, as well as dissolution / elimination processes of protective metal oxides in the ones on the pipe wall forming sulfate and chloride melts, whereby a Passivation of the pipe material additionally difficult will, together. In such conditions point yourself high-alloy steels only unsatisfactory Corrosion resistance.

More recent developments show that pipes are made Nickel-based materials have increased resistance exhibit against these corrosion processes, due to the high price to be paid for nickel, however, is that Apply a corrosion resistant ferritic Coating on low-alloy steel pipes cheaper alternative.

The use of coated pipes in plants for thermal waste disposal has the advantage that the mechanically stressed pipe cross-section low-alloy steel and the component after thermal / mechanical criteria can be designed. The materials with the required high Corrosion resistance are only considered comparative thin layer used on the pipe surface. The Requirements for the mechanical properties of the Coating material are low, so that at its selection only the corrosion resistance and the Adhesion to the low-alloy substrate is crucial are.

The coating of corrosively highly stressed Heat exchanger tubes in such systems are used primarily for Repair of corrosion damage as well as for prevention critical positions, already done. This will on known materials or derived from them Variants used, such as Alloy 625, one Nickel based alloy with 21.5% chromium, 9% molybdenum, 3.6% niobium and 2.5% iron and smaller proportions Al, Ti, Mn, Si and C, or NiCrBSi solders, the im Compared to iron higher resistance of nickel against chlorine corrosion.

In addition to the prior art explained above from US 5,643,531 a process for the production of Protective coverings for pipes in feed water preheaters from industrial furnaces, which temperatures from can be exposed up to 400 ° C, known. In doing so a reduction in erosive erosion due to the Contact of the pipes with the large proportion of fly ash in the Exhaust gas can be reached. To do this, a layer is made from a Steel powder applied to the pipes, in addition to iron the elements chromium, manganese, carbon, Prescribes silicon, molybdenum and aluminum. The Aluminum content is 3% to 13%. by virtue of this high aluminum content occurs in the known Coating to form hard phases that the Give the material the desired wear resistance. At the same time form those known from US 5,643,531 Alloys due to the presence of aluminum, however at low and medium temperatures Crystal transformation an intermetallic compound (Iron aluminide), which is both brittle and by their volume change associated with the conversion Micro cracks and tension in the structure of the coating produce. This leads to a reduction in Corrosion resistance.

The invention is based on the idea that also ferritic steel alloys can be used provided that their chromium and silicon contents are sufficiently high are, so that the formation is very stable, the material passivating oxides also among those described above Corrosion conditions are preferred. The advantage such ferritic coatings lies in their low-alloy substrate material practically the same thermal expansion, which also ensures adhesion Thermal cycling is guaranteed. The high thermal conductivity of ferritic steel has an advantageous effect on heat transfer and therefore the surface temperature of the coating.

mehr als 20 bis 50 % Cr

5 bis 10 % Si

Rest Fe, einschließlich erschmelzungsbedingter Verunreinigung sowie fakultativen Zusätzen von

bis 4 % B

bis 5 % Mn

bis 1 % Mo

bis 0,1 % C,

Purely ferritic iron-based alloys which, according to the experiments carried out, can be considered as such a coating material have a composition of (in% by mass)

more than 20 to 50% Cr

5 to 10% Si

Rest of Fe, including melting contamination and optional additions of

up to 4% B

up to 5% Mn

up to 1% Mo

up to 0.1% C,

the lowering of the melting temperature and thus the Serve to improve order properties.

Steels with less than 20% chromium and less than 3% Silicon does not meet the requirements for corrosion resistance towards the media mentioned. More than 50% chromium showed no further improvement in terms Corrosion resistance, only increases the cost of steel. at a content of more than 10% silicon deteriorated again the corrosion resistance to the mentioned media. Optimal alloy contents were included 25 to 50% chromium and 5 to 8% silicon determined.

Fe-35Cr-5Si

Fe-30Cr-5Si

Fe-30Cr-5Si-1B

den höchsten Korrosionswiderstand auf.According to the experimental test results to date, the alloys

Fe-35Cr-5Si

Fe-30Cr-5Si

Fe-30Cr-5Si-1B

the highest corrosion resistance.

Such coatings can be found in all plants in which gases containing chlorine and / or chloride and / or sulfate-containing dusts and deposits Cause corrosion problems, e.g. when burning Household waste, biomass and chemical waste as well as at Copper refining, used promisingly.

example

The corrosion resistance of the alloys has been confirmed in aging experiments at 600 ° C. The samples of the alloys examined were completely embedded in deposits that come from the heat exchanger tubes of a waste incineration plant. The deposits were renewed every 2 weeks. The reaction gas consisted of 5 vol.% O ₂ containing nitrogen, to which 500 vol. Ppm HCl had been added. The water content of the gases dried in a P ₂ O ₅ column was <3 × 10 ^-5 mbar.

The samples have been exposed to these conditions for various times (previously up to a maximum of 8 weeks). The loss of material due to corrosion has been determined mechanically and chemically by pickling in alkaline KMnO ₄ solution and inhibited hydrochloric acid on the basis of the decrease in mass after removal of the corrosion products and deposits.

The time-dependent material losses .DELTA.m due to corrosion determined in this way are plotted in FIG. 1 for various FeCrSi alloys and for Alloy 625. The corrosion rates of the FeCrSi alloys used according to the invention are up to an order of magnitude lower than that of Alloy 625, the material which is now predominantly used for coating. The comparative alloy Fe-30Cr-2Si, which therefore contains less silicon than the steels to be used according to the invention, also has a significantly higher decrease in mass of 0.06 mg / mm ² after 14 days.

With the described method for determining the Material losses can only be lost over the entire Record sample surface averaged values. Among the However, under the conditions examined, local corrosion is too watch that in the areas affected by this too. a larger cross-sectional decrease than that from the area-specific mass decrease calculated leads. at the determination of a minimum thickness to be required The coating to be applied is subject to local corrosion take into account as the passage of the corrosive medium at such a point for failure of the entire pipe can lead.

The local corrosion can be determined when the Take the minimum layer thickness into account by adding the am Cross section of the corrosive sample determined maximum Depth of the dimples in the metal surface from the area-specific mass change calculated cross-sectional decrease is added.

After 8 weeks of aging under simulated Waste incineration conditions are from Mass decreases calculated cross-sectional decrease of Fe-Cr-Si alloys to be used according to the invention only at about 0.02 mm. The depth of due to local corrosion resulting dimple is about 0.03 mm, so for this period with a maximum decrease in cross-section of 0.05 mm is to be expected. In comparison, the reduction in cross section calculated from the change in mass Alloy 625 in the same period 0.24 mm. The local Corrosion is added with a surcharge of about 0.04 mm consider.

Powder made from the FeCrSi alloy to be used according to the invention can be made by direct Atomizing the alloy melt or by induction drip melting rod-shaped electrodes from the corresponding alloy. The latter procedure was with Success in producing smaller amounts of Fe-35Cr-5Si powder applied. The application methods are thermal Spraying process provided. Due to requirements the tightness of the layer comes here in particular Flame spraying in question. Using high-speed flame spraying have already successfully applied coatings be performed, which also the desired Ensure protection against corrosion.

Claims (3)

1. A use of powder of a ferritic steel alloy having (in mass-percent)

   more than 20 to 50 % chromium,

   5 to 10 % silicon,

   as well as alternately

   up to 4 % B,

   up to 5 % Mn,

   up to 1 % Mo,

   up to 0.1 % C,

   and the remainder iron, including unavoidable impurities,
   for thermal coating of metal parts which are subjected during operation to corrosion attacks by hot media containing chlorine and/or chloride and/or sulfate.
2. A use according to claim 1, characterized in that the steel alloy (in mass-percent) includes 25 to 50 % chromium and 5 to 8 % silicon.
3. A use according to claim 1 or 2, characterized in that the components are facilities for thermal waste disposal or for refining copper.

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