GB2145115A

GB2145115A - Corrosion resistant materials

Info

Publication number: GB2145115A
Application number: GB08321967A
Authority: GB
Inventors: William Michael Cox; David Barry Meadowcraft
Original assignee: UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY
Current assignee: UNIVERSITY OF MANCHESTER INSTITUTE OF SCIENCE AND TECHNOLOGY
Priority date: 1983-08-16
Filing date: 1983-08-16
Publication date: 1985-03-20
Also published as: GB8321967D0; GB2145115B

Abstract

A low alloy steel appreciably more resistant to acid dew point corrosion than mild steel contains 1 to 8% nickel, 0.5 to 3% chromium and 0.1 to 1% molybdenum, the balance being iron and non- interfering elements. This low alloy steel can be used for plant or equipment exposed to acid dew point corrosion, for example combustion plant.

Description

SPECIFICATION Corrosion resistant materials This invention concerns corrosion resistant materials, more particularly it concerns materials which are resistant to dew point corrosion.

In many industrial processes, hot gases containing acidic components come into contact with metal surfaces at temperatures at which liquid acids and moisture condense out.

This provides an aggressive environment for the metals and the corrosion which occurs is termed "dew point corrosion". Dew point corrosion is a serious problem, especially in waste gas sections of plants burning fuels containing sulphur, and causes considerable expense in equipment degradation, plant shutdowns etc. The most active agent in causing dew point corrosion is sulphuric acid, although hydrogen chloride is often present alone or in addition. Metal surfaces in such plants are frequently of mild steel for reasons of cost, and mild steel is susceptible to dew point corrosion. The conventional engineering solution is to increase the gauge of the mild steel used, thus postponing the failure of the metal part, and/or to discharge gases at relatively high temperature, with concomitant loss of thermal efficiency.

Dew point corrosion occurs only at relatively low temperatures, at which the acid condenses out of the gas carrying it, and may at one atmosphere ambient pressure be generally said to occur at temperatures less than 200"C; such corrosion is frequently most serious at temperatures in the range 90 to 1 50 C, especially 100 to 1 20 C.

As part of an extensive study of this problem, involving the assessment of alioys, coatings and treatments, we have discovered that certain low alloy steels demonstrate unusual resistancevto dew point corrosion. It was to be expected that high alloy steels would show resistance since these are frequently recommended for general corrosion resistance, but it was found that the properties of the steels tested varied considerably when exposed to dew point corrosion conditions, and that while some high alloy steels did exhibit corrosion rates'appreciably less than mild steel, others exhibited almost identical rates to mild steel.

A low alloy steel has now been identified which consistently exhibited appreciable long term resistance to acid dew point corrosion.

This low alloy steel falls within a steel standard specification for case-hardened steels, and it is believed that this steel or the specification has previously not been identified as possessing any unusual resistance to corrosion.

Accordingly, the present invention provides the use of a low alloy steel containing 1.0 to 8.0% by wt. of nickel, 0.5 to 3% by wt. of chromium and 0.1 to 1.0% by wt. of molybdenum, the balance being iron and other elements in amounts which do not significantly adversely affect the properties of said steel, in plant or equipment exposed to acid dew point corrosion.

The invention also provides metal plant or equipment exposed to acid dew point corrosion, wherein at least the metal surfaces in contact with the acid are of a low alloy steel containing 1.0 to 8.0% by wt. of nickel, 0.5 to 3% by wt. of chromium and 0.1 to 1.0% by wt. of molybdenum, the balance being iron and other elements in amounts which do not significantly adversely affect the properties of said steel.

In general, the plant and equipment which may be advantageously made according to the invention of the specified steel are those normally at temperatures of less than 200"C during operation, and may be found in plants involved in the combustion of materials containing sulphur. Such plants may be for the combustion of coals of all types and coalderived fuels, petroleum oils including fuel oils and waste streams, the combustion of hydrocarbon gases, and incineration of solid, liquid or gaseous wastes including domestic and industrial refuse, with or without added fuel, and conversion processes yielding acid gases. Major uses are expected to be in electricity generation by fuel combustion, by major utilities down to local or plant generation and stream or process heat generation on site.The equipment which may be advantageously made of the low alloy steel according to the invention includes heat exchange surfaces such as economisers, air pre-heaters, metal waste gas stacks, gas cleaning equipment such as electrostatic precipitators, cyclones and bag filter housings, ducting, fans, dampers, instrumentation housings and sheaths and fittings usually found in the low temperature end of combustion and incineration equipment.

The low alloy steels which are the subject of this invention may be made and processed in known manner, although the steels do exhibit work and air hardening, so that account of this should be taken during processing. The other elements which may be present in the steels are those customarily found in steels, such as 0.1 to 0.2% by wt. of carbon, 0.2 to 0.6% by wt. of manganese and 0.1 to 0.5% by wt of silicon. Elements such as copper, aluminium and sulphur may be present in amounts usually not exceeding 0.05%, but the real criterion is the long term resistance of the steel against acid dew point corrosion, combined, of course, with adequate mechanical and structural properties.

The steels preferably contain 3.5 to 4.5% nickel, 0.75 to 1.25% chromium, 0.25 to 0.4% manganese and 0.1 to 0.35% silicon and 0.2 to 0.4% molybdenum. Steels of such analyses are found in standards such as the old EN39B British standard, BS 970 Part 3 Grade 835 A15 and Japanese JIS Grade SNCM 25. Nonetheless, in Britain at last such steels are not commercially made for this application.

The steels the subject of this invention were discovered to have the desired properties by exposure of samples of different alloys in a laboratory combustion test rig. This test rig is fired by mains natural gas and the samples are exposed to pre-selected dewpoint conditions. Sulphur dioxide is injected into the combustion chamber at a controlled rate, so that desired amounts of sulphur trioxide are generated in situ, and the amount of sulphur trioxide is accurately regulated by the excess oxygen-in the flue gas and is monitored by a Severn Science/MEL continuously recording SO3 monitor. Hydrogen chloride gas may be injected in addition, but this does not appear to have a significant affect at higher dew point temperatures typical of modern combustion plant dry exhaust systems.

After the desired length of exposure to the dew point conditions the metal samples are removed and corrosion products are removed by treatment with inhibited dilute hydrochloric acid. This acid treatment causes a negligible weight loss to the uncorroded remaining metal. The primary method of corrosion rate estimation in laboratory work has been weight loss measurements, with the samples removed at periods up to two weeks.

Corrosion rates for mild steel have been estimated to be 1 .7mm per year and for the low alloy steel of the invention, 0.81 mm/ year, under exposure conditions of 1 Sppm 803, 1000 ppm 202, 800ppm HCI, 1.3% excess 02 and a sample temperature of 1 20"C. For comparison, the commercial alloy "HASTALLOY" B2 has an estimated corrosion rate of 0.40mm/year but this alloy is extremely expensive and it would not be economic or practicable to construct any plant or equipment except perhaps small and vital pieces of equipment.

Our studies have shown that the low alloy steel demonstrates a similar corrosion to mild steel for the first 35 hours of exposure, with most of the corrosion occurring in the first 24 hours. Mild steel then shows a continuing constant rate of corrosion whereas the low alloy steel shows a decrease in corrosion rate, indicating that a different corrosion mechanism occurs although this is not fully understood at this time. After 350 hr exposure to standard test conditions, the metal loss for the low alloy steel is typically less than one half that for mild steel. It is therefore clear that for long term resistance to acid dew point corrosion, the low alloy steel is appreciably better than mild steel.

Claims

1. The use of a low alloy steel containing 1.0 to 8.0% by wt. of nickel, 0.5 to 3% by wt. of chromium and 0.1 to 1.0% by wt. of molybdenum, the balance being iron and other elements in amounts which do not significantly adversely affect the properties of said steel, in plant or equipment exposed to acid dew point corrosion.

2. The use of claim 1, where in the low alloy steel contains 3.5 to 4.5% nickel, 0.75 to 1.25% chromium, 0.2 to 0.4% molybdenum, 0.25 to 0.4% manganese and 0.1 to 0.35% silicon, the balance being iron and other elements in amounts which do not significantly adversely affect the properties of said steel.

3. The use of cliam 1 or 2, in plant or equipment exposed to acid dew point corrosion and exposed to temperatures of less than 200"C during operation.

4. The use of claim 1, substantially as hereinbefore described.

5. Metal plant or equipment exposed to acid dew point corrosion, wherein at least the metal surfaces in contact with the acid are of a low alloy steel as described in claim 1 or 2.