Figure 1

The influence of the Mo/C ratio in different heats of low-alloy steels on the total coating thickness of the carbide toplayer.The best diffusion coatings were tested using four criteria that reflect in-service reliability.

Chromium Difusion Coatings for the Protection of Low-alloy Steel in a Sulphidizing Atmosphere.

Corrosion Science 1993, Nos 5-8, pp 1235-1242. (paper 44)

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The chromium diffusion process was investigated for carbon-, low-alloy and austenitic type steels. The quality of the diffusion coatings on low-alloy steels was tested using four criteria that reflect in-service reliability of heat exchangers in coal gasifiers: thermal stability, ductility, resistance to down-time corrosion and weldability.

These tests have shown that good quality diffusion coatings can be made on 10CrMo910. The corrosion resistance of the chromium diffusion coatings can be greatly enhanced by the presence of small amounts of carbide formers like vanadium and niobium.

In new combustion techniques such as low-NOx combustion and coal gasification, the firing atmosphere will be relatively reducing1 (PO2 approx. 10-23 Pa and pS2 approx. 10-4 Pa). In these circumstances, a protective oxide layer may not be formed and the evaporator tubes have to be protected against the aggressive environment by other means. Chromium diffusion coatings can be used to prevent corrosion in such a reducing atmosphere.

The chromium diffusion coatings are applied with the pack cementation process (chromizing). In this process the objects to be coated are placed in a fine powder mixture (i.e. the pack). This powder consists of an inert filler (AI₂O₃), an activator (usually NH₄Cl) and chromium.

During heating of the pack, the activator is cracked into hydrogen chloride (HCl), N₂ and H₂. The HCl formed reacts with chromium according to:

2HCl(g) + Cr(s) --> CrCI₂(1) + H₂(g).

Besides, a small amount of CrCl₃ is formed as well. During the pack cementation the partial pressures of the chromium chlorides (CrCl_x) are sufficiently high to transfer them via the gas phase to the steel surface. There the CrClx dissociates into metallic Cr and HCl, according to:

CrClx(g) + xH₂(g) --> Cr(s) + 2xHCl(g).

The metallic Cr deposited at the steel surface diffuses into the metal and the HCl is subsequently transferred back to the chromium powder in the pack where it reacts to form CrCl_x.

This concludes the cycle of Cl in the gas phase.

At approximately 1100 °C the strong interaction between the chromium and the free carbon present in the steel results in the formation of chromium carbides at the metal surface. These carbides act as a diffusion barrier and limit the growth of a metallic chromium-rich top layer necessary for corrosion protection. Under the coating a decarburized zone will develop.

With the use of strong carbide formers the formation of the chromium carbides can be prevented, These carbide formers should either be present in the steel (e.g. small quantities of niobium) or can be added to the pack (e.g. vanadium). The large number of parameters in both the pack cementation conditions and the steel composition makes a prediction of the coating quality virtually impossible.

• In this work the coating process was investigated for diffusion coatings on carbon and low-alloy and austenitic type steels:
• The evaluation of the long term stability of the coating / steel system at operating temperatures (approx. 400°C).
• The temperature fluctuations that occur during start-up and shut-down procedures will result in the formation of high tensile stresses and deformation in the material. Low strain-rate deformation in a sulfidizing atmosphere can lead to a specific form of stress-corrosion in evaporator tubes known as Alligator Skin Cracking (ASC). The coating should also be resistant against high strain-rate deformation.
• Resistance against down-time corrosion (DTC). During a shut-down period the relative humidity can exceed the dew point causing hygroscopic salts to create a very corrosive liquid on the metal surface
• The weldability of coated steel and the coatability of a weId must be evaluated.

The main conclusions to be drawn from this study on Cr and CrV diffusion coatings for evaporators that operate in a sulphidizing atmosphere are that:

• Carbon steel, austenitic steel (AISI 310) and low-alloy steel where carbon is only partly stabilized (15 Mo 3, 13 CrMo 4 4) are unsuited as base material for conventional Cr and CrV diffusion coatings.
• Cr diffusion coatings on low alloy steel 10 CrMo 910 are sensitized and therefore they are sensitive to corrosion.
• Cr and CrV diffusion coatings on 10 CrMoNb 910 and to a somewhat lesser extent CrV coatings on 10 CrMo 910 without Nb) seem to be an excellent choice. These diffusion coatings will therefore be included in large scale experiments to be carried out in test installations.