GB2279966A

GB2279966A - Improving alloy compositions

Info

Publication number: GB2279966A
Application number: GB9314821A
Authority: GB
Inventors: Lee Shaw
Original assignee: Special Melted Products Ltd
Current assignee: Special Melted Products Ltd
Priority date: 1993-07-17
Filing date: 1993-07-17
Publication date: 1995-01-18
Also published as: GB9314821D0

Abstract

An alloy preferably containing chromium is treated by the addition of rare earth in an amount by weight equal to between 2 and 30 times the sum of the sulphur and oxygen contents of the alloy. The alloy so treated has enhanced pitting corrosion resistance.

Description

Improvements in and Relating to Alloys having Enhanced Pitting Corrosion Resistance This invention relates to alloys having enhanced pitting corrosion resistance. More especially, but not exclusively, the invention relates to chromium containing alloys and steels having enhanced pitting corrosion resistance.

It is known that the presence of sulphide inclusions in alloys can lead to pit initiation and that the addition of titanium results in the formation of titanium containing inclusions with consequent improvements in corrosion performance. The presence of significant levels of titanium in certain chromium containing steels, however, is known deleteriously to effect mechanical properties.

The present invention in one aspect sets out to provide a method of treating alloys by discrete additions of rare earth metals to provide for those alloys enhanced pitting corrosion resistance. The present invention also sets out to provide alloys treated with rare earth metals having enhanced pitting corrosion resistance with no apparent adverse effect on the mechanical properties of such alloys.

According to the present invention in one aspect there is provided an alloy to which has been added a rare earth in an amount by weight equal to 2 and 30 times of the sum of the sulphur and oxygen contents of the alloy.

The addition of rare earth may be between 2 and 20 times; or between 3 and 15 times; or between 5 and 15 times; or between 8 and 10 times the sum of the sulphur and oxygen contents of the alloy.

A typical addition is approximately 10 times the sum by weight of the sulphur and oxygen contents of the alloy.

In another aspect the present invention provides a method of treating a chromium-containing alloy in which rare earth is added to a melt of the alloy in an amount equivalent to between 2 and 30 times by weight of the sulphur and oxygen contents of the alloy.

The rare earth metal may comprise mischmetall, cerium or lanthanum. Mischmetall is an alloy of rare earths which generally contains approximately 45% cerium, 35% lanthanum, neodymium and praseodymium, and approximately 20% of samarium, erbium, gadolinium and yttrium.

The alloy is preferably a chromium-containing alloy or steel having, before treatment, a sulphur content of between 0.001 and 0.07% by weight. The sulphur content of the alloy may be in the range 0.001 to 0.03% by weight; alternatively, between 0.001 and 0.015% by weight. A typical sulphur content is 0.005% by weight. The oxygen content of the alloy may be in the range 0.001 to 0.01% by weight. Typically, the oxygen content is 0.003% by weight.

The invention will now be described by way of example only with reference to the following example of an alloy treatment in accordance with the invention.

In this example, a melt of a chromium containing alloy of composition by weight % of 0.09 C, 0.40 Si, 0.83 Mn, 0.19 P, 0.002 S, 10.38 Cr, 0.67 Mo, 0.50 Ni, 0.015 Al, 0.0055 B, 5.29 Co, 0.02 Cu, 0.023 N, 0.32 Nb, < 0.0001 Pb, 0.001 Su, 0.007 Ti, 0.22 V and 0.01 W. To this melt an addition of 0.1% by weight mischmetall was made. This addition represented approximately 20 times the sum by weight of the sulphur and oxygen contents of the alloy.

Test electrodes were prepared from the solidified alloy and immersed in a solution of 1000 ppm chloride ion at ambient temperature (20"C) and 60"C. Each test electrode was immersed in a separate solution of 1000 ppm chloride ion made up in deionised water. The test solutions were naturally aerated. The potential of each electrode was first monitored until stable typically over a period of approximately one hour.At the end of this stabilisation period the electrodes were held at a potential of -800mV relative to a saturated calomel electrode for one minute to cathodically clean the surface, then polarised to potentials of +300mV and back to -800mV relative to the saturated calomel electrode at a scan rate of lmV/s. Measurement and recording of the potential and current changes during polarisation were performed using an electrode chemical interface driven by software.

At 20"C, no corrosion of the electrode surface was observed during the stabilisation period. The rest potential was measured at minus 161mV. Polarisation of the electrode to anodic potentials resulted in the establishment of a constant current as the potential was raised in the positive direction. This passive range extended from approximately minus 300mV to plus 300mV.

At 600C no corrosion of the electrode surface was observed during the stabilisation period. The rest potential was measured at -9lmV. Polarisation of the electrode to anodic potentials resulted in a very unstable passive region displaying wide variations in a passive current density. This region extended from approximately -300mV to +200mV at which point the current density sharply increased reaching a maximum of 1.5mA/cm2 at a potential of +300mV.

Thus the treated chromium containing alloy samples showed no sign of pitting at their free potential at either ambient temperature (20"C) or 60"C. Subsequent polarisation to anodic potentials indicated passive-transpassive behaviour, that is protection by an oxidelfilm with subsequent breakdown high potentials.

The amount of rare earth added to the alloy must be sufficient to react with the sulphur present in the alloy to produce stable rare earth sulphides which are not susceptible to pitting corrosion. As a result of the treatment the sulphur content of the treated alloy may or may not be reduced. In either event, the sulphur content of the alloy is essentially stabilised in a rare earth sulphide form. In practice, it has been established that an addition of at least twice the sum by weight of the sulphur and oxygen contents of the alloy is necessary to achieve the desired effect. Typically, an addition of between 2% and 30% by weight is made; a preferred addition is between 8 and 10 times the aggregate of the sulphur and oxygen contents. The maximum rare earth addition is one which does not adversely affect the mechanical properties of the alloy. This has been found to be 30 times the sum of the sulphur and oxygen contents of the alloy; a preferred maximum is 20 times the aggregate of the sulphur and oxygen contents.

Alloys produced in accordance with the invention have contents of rare earth signficantly less than the additions made during treatment.

It has been found that the weight loss due to corrosion of alloys treated with rare earth in accordance with this invention is approximately 10% the weight loss due to corrosion of untreated alloys of the same composition.

It will be understood that the following example is merely exemplary of alloys and treatments thereof in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention.

Claims

1. An alloy to which has be added a rare earth in an amount by weight equal to between 2 and 30 times the sum of the sulphur and oxygen content of the alloy.

2. An alloy as claimed in claim 1 in which the by weight addition of rare earth is between 2 and 20 times the sum of the sulphur and oxygen contents of the alloy.

3. An alloy as claimed in claim 1 wherein the by weight addition of rare earth is between 3 and 15 times the sum of the sulphur and oxygen contents of the alloy.

4. An alloy as claimed in claim 1 in which the by weight addition of rare earth is between 5 and 15 times the sum of the sulphur and oxygen contents of the alloy.

5. An alloy as claimed in claim 1 in which the by weight addition of rare earth is between 8 and 10 times the sum of the sulphur and oxygen contents of the alloy.

6. An alloy as claimed in any one of claims 1 to 5 wherein the rare earth metal comprises mischmetall, cerium or lanthanum.

7. A chromium containing alloy having a sulphur content of between 0.001 and 0.07% by weight to which has been added a rare earth in an amount equal to that claimed in any one of claims 1 to 5.

8. An alloy as claimed in claim 7 wherein the sulphur content of the alloy is in the range 0.001 to 0.03 by weight.

9. An alloy as claimed in claim 7 wherein the sulphur content is in the range of 0.01 to 0.015 by weight.

10. An alloy as claimed in claim 7 whose oxygen content is before the addition of rare earth in the range of 0.001 to 0.01% by weight.

11. A method of treating a chromium containing alloy in which rare earth is added to a melt of the alloy in an amount equivalent to between 2 and 30 times by weight of the sulphur and oxygen contents of the alloy.

12. A method as claimed in claim 11 wherein the by weight addition of rare earth is between 2 and 20 times the sum of the sulphur and oxygen contents of the alloy.

13. A method as claimed in claim 11 in which the addition by weight of rare earth is between 3 and 15 times the sum of the sulphur and oxygen contents of the alloy.

14. A method as claimed in claim 11 wherein the by weight addition of rare earth is between 5 and 15 times the sum of the sulphur and oxygen content of the alloy.

15. A method as claimed in claim 11 wherein the by weight addition of rare earth is between 8 and 10 times the sum of the sulphur oxygen contents of the alloy.

16. The chromium containing alloy as described.

17. A method of treating a chromium container alloy as described.