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[ EN ]

Method of analysing carbon and carbon compounds.

The present invention relates to a method and apparatus for selective analysis of carbon and carbon compounds on heat resistant materials and compositions of materials in respect of their content of carbon and hydrogen. The method is particularly suited to the determination of oil, greese and carbon residues on the surface of different inorganic objects, e.g. metallic objects. It is of importance to know the quantity and kind of substances on the surface of objects, particularly carbon and carbon compounds, since they can have a large effect, e.g. on corrosion properties and the possibility of applying surface coatings.

In surface coating it is particularly of great importance that objects are very well cleaned before the surface is coated. A poorly cleaned metal surface often leads to the adhesion ability of the surface coating behing reduced and the properties of the surface-coated product being deteriorated. There .- as a large technical and industrial need of quantitati' 3. qualitative determination of surface contaminations on metal surfaces.

In cold forming, such as rolling, wire and tube drawing of steel, copper and copper alloys, aluminium and aluminium alloys and other metals as well, different lubricants are used to reduce friction against the tool. After cold forming, the metal surfaces are usually cleaned by degreesing and pickling or annealing the routine determination and control of the effects of different cleaning methods is also of great interest here.

It has been found that the methods used for routine surface analysis of carbon and carbon compounds on metal surfaces do not give satisfactory results. The present invention therefore covers an important technical and industrial requirement.

Methods and apparatus for analysing carbon residues on metal surfaces have been developed lately (1979), a technique being utilized in which carbon and carbon compounds are oxidized in pure oxygen gas at 180 C.and then again at 600 C with subsequent oxidation of the formed volatile carbon compounds with copper (II) oxide or barium chromate, the carbon dioxide formed being measured coulo etrically or with a variety of known analytic methods. With this technique one can differentiate between pure carbon and organic compounds which are easy to drive off.

In organic elementary micro analysis information may be obtained as to the exact quantity ratios between different ingoing elements,- which means that the total quantity e.g. of carbon or hydrogen is measured using gravimetric, coulo-metric or other suitable methods. Oxidation always takes place at high. temperature so that the entire sample is completely combusted.

In accordance with the present invention there is proposed a novel method of analysing carbon and carbon compounds on the surface of objects by oxidation in the presence of a passing gas containing oxygen, with subsequent determination of the water vapor and carbon dioxide. content of the gas. The method is characterized by -the material being placed in a first oven in which the temperature is raised at a regulated rate of 0,05 to 10 Cs and by the content of water vapor. and carbon dioxide of the passing gas being registered continuously with respect to the quantity and in relation to the temperature in the first oven. Before registration the gas is caused to pass through a second oven containing an oxidation catalyst at a temperature over 500 C, after which the gas is cooled to a suitable constant temperature at which registration can be performed.

The novel method enables selective analysis of carbon and the nature of the carbon compounds, which is of great import- ance for industrial applications, where oil residues, for example, may present completely different problems from graphite residues in such surface coatings of metals as enameling.

In controlled heating of an object, volatile carbon compounds will first be vaporized at different temperatures depending on the composition of the compound. The vaporized compound will be entrained by the gas and taken over to the second oven where carbon and hydrogen in the vaporized carbon compound will be reduced to water vapor and carbon dioxide.

The gas containing water vapor and carbon dioxide thus obr tained is then cooled to a lower constant temperature.
This temperature is decided by what temperature can be accepted when the water vapor and carbon dioxide are registered. As the temperature increases, different carbon compounds with different volatility will be successively vaporized and oxidized and transferred by the carrier gas to the apparatus for registering water vapor and carbon dioxide. At high temperature carbon in the form of graphite will be directly oxidized and carbon monoxide thus formed will be transformed into carbon dioxide in the second oven, which is maintained at constant high temperature.

Registration of the water and carbon dioxide contents of the gas can be performed in several ways known per se.

Measurement of the water quantity can be . made by coulo-metric measurement, a Keidel cell. Measurement may also be made by measuring .the conductivity, infrared
absorption or .variations in capacitance.

Measurement of the carbon dioxide content can be made with similar methods, such as conductometry, coulometry or infrared absorption technique.

O PI If conduciϋometry is used, a carbon dioxide-containing gas - is taken down into a sodium hydroxide solution, for example, and variations in conductivity measured. Using carbon or calcium carbide, water can be converted to an equivalent quantity of carbon dioxide and measured as such.

With coulometry changes in the pH value can be measured if the carbon dioxide is taken down into a barium perchlorate solution, information as to the quantity of carbon dioxide in the gas thus being provided. Water vapor may be lead into a Keidel cell, in which platinum wires with phosphor pentoxide electrolize water to hydrogen and oxygen at a given applied voltage. The measured electrolysis current corresponds to the quantity of water vapor. also possible to measure infrared absorption at given wavelengths for gases flowing out from oven or
furnace, chambers. .

Water vapor may be continuously measured with a dew-point- meter by measuring changes in conductivity or capacitance across an aluminium oxide layer.

All the foregoing described measuring methods may be per-• formed with commersially available instruments, amongst which may be mentioned gas cromatographs measuring thermal conductivity, e.g. Hewlett Packard Model HP 185. Keidel cells are manufactured b>y Salzer-Labor atik/Wδshleff.
Apparatus for infrared absorption are manufactured by
Leybold—Heraeus and Beckman Instruments, while apparatus for measuring changes in conductivity or capacitance are made by Endress-Hanser G bH+'Co and Moisture Control &
Measurement Ltd (MCM) .

The invention also includes an apparatus for carrying out the inventive method. In its simplest form, such apparatus comprises a first oven provided with heating means controlled by a regulating device, and with an oven chamber for
O PI the passage of a gas containing oxygen, there also being means for supplying oxygen-containing gas and the insertion of a test object. By "oxygen-containing" gas is intended a gas containing at least 8 % molecular oxygen. The appara- tus also includes a second oven in which a constant high temperature can be maintained at between 500 and 1200 C, the oven chamber here being provided with an oxidation catalyst, the nature of which determines the appropriate temperature, and which may comprise copper (II) oxide,
barium chromate, cobolt (II",III) oxide, dissolution products of silver permanganate, nickel oxide or tin (IV) oxide.

The first and second ovens may also be put together, it .then being necessary. to make sure that there is sufficient insulation between appropriate parts of the ovens.

The oven space in both ovens or oven parts suitably corn- prices an inert or resistant material such as quarts glass tubes or other material having corresponding resistance, in which a sample holder of inert material can be placed. Alternatively, the test object can be placed directly in the quarts glass tube. Means for the supply of oxygen-containing gas are arranged outside the heat zones in the . ovens so that. the gas is caused. to flow through both ovens or oven parts. Instead of quarts glass, . certain types of stainless steel, may be used to .advantage, especially such steel, having a high chromium content.

According to one embodiment, the oven space is formed as a chamber to which inert gas is .supplied. Departing inert gas is mixed with oxygen containing gas in another space in the oven, the total gas mixture being then transferred to the second oven containing the oxidation catalyst, where the entire quantity of carbon is turned into carbon dioxide and the hydrogen present is turned into water vapor.

O PI S WIPO More information can be obtained if the analysis is supplemented by a method where the test sample is protected from contact with the oxygen-containing gas during heating to about 800 C with inert gas, the aforementioned embodiment being utilized. By the test sample "being heated in the absence of oxygen possible volatile compounds will be vaporized and possibly cracked, subsequent to which they are transferred to another chamber and mixed in gas form there with oxygen-containing gas and are oxidized completely.

It is. also possible to heat the test object to 800 C in inert gas first, and subsequently cool it to a temperature under 200 C and finally raise the temperature at a regulated rate with oxygen-containing gas flowing in contact with the test object. In this way the oxidation of non-volatile compounds can be separated from compounds which are volatile at the same temperature.

It is, of course, also possible to analyze the gas from the ovens in respect of other substances and compounds in the cases where it is desired to obtain further information.

Gas is taken from the second oven via a conduit to a- cooling means for indirect cooling to about room temperature and further to both detectors for. water and carbon dioxide.
The conduit is suitably provided with a flowmeter, possibly coupled to a pen recorder. Output data from the detectors is registered conventionally on a pen writer or in some other way, and on an integrating meter for obtaining content totals if so desired.

The result obtained will be denoted on a recorder as the registered quantity of water and carbon dioxide, respectively, in relation, to the time or temperature in the first oven. The graph for carbon dioxide will have crests corresponding to the vaporization or oxidation temperature for carbon and carbon compounds in the first oven. In a corresponding way.

hydrogen liberated from the test object will be registered as water at the vaporization or oxidation temperature for hydrocarbon compounds or molecular hydrogen that are possibly present.

After some trial activity, one skilled in the art can read from the graphs obtained particulars about carbon and carbon compounds on the test objects, which has not been possible with previously known testing methods and testing equipment.

The invention will now be described in detail with reference to the .accompanying drawings illustrating preferred embodiments of the apparatus in accordance with the invention and whereon
Figure 1 schematically illustrates an apparatus in accordanc with the invention.
Figure 2 illustrates the implementation in detail of the oven chamber in a first oven,
Figure 3 an example of a graph obtained on a pen recorder, and
Figure 4 an example of a graph obtained if the test object is heated in an inert atmosphere.

Figure 1 illustrates a first oven 1 provided with a control unit 2, regulating via a line 3 the temperature rise in the oven 1 to 0,05 - 10 Cs , and a second oven 4 regulated by a control unit 5. via a line 6. The oven chamber in the first oven 1 comprises a tube 8 provided with an* inlet 9 for oxygen-containing gas, and a removable closer means 10 in which an aperture 11 serves as a, pressure regulator.
In the oven chamber 7 there is indicated a sample holder 12 which can be inserted into the oven chamber- 7 after removing the removable closer means 10. When in use, supplied gas is caused to flow through the oven chamber 7 at a rate such that the gas volume in the chamber is -changed 1-60
times per mm . The oven chamber 7 is defined relative tl oven chamber 13 in the second, oven 4 by a constriction 14 in the form of a glass filter. A bed 15 of oxidation catalyst is placed in the oven chamber 1 . A constant temperature of about 600°C is kept in the oven 4 when barium chromate is used as an oxidation catalyst. The oven chamber 13 extends outside the oven 4 to a cooling space 16. The gas is cooled to room temperature in this space. After cooling the gas is lead consecutively to a flow eter 18 via a conduit 17, a water detector 20 via conduit 19 to a carbon dioxide detector 22 via a conduit 21 and finally to the atmosphere via a conduit 23. The conduit 23 may be provided with a non-return valve (not shown) . Measurement signals are taken via the lines 24, 25, 26 and 27 to a pen writer 28 and to an integrating device 29 with a digital display.

The oven chamber 7 in Figure 1 is illustrated in Figure 2. The tube 8 of Figure 1 is provided with a tapering portion 30 and a discharge outlet 31 through which the gas will be taken to an oven chamber corresponding to the oven chamber 13 of Figure 1. In the interior of the tube 8 there is arranged the tubular portion 32 with a bevel 33 snuggly fitting the tapering portion 30 of the tube 8. At its end remote from the bevel 33 the part 32 is provided with a filter 34, in the centre of which there is adapted a tube 35 provided with an expanded portion 36 at its end remote from the part 32. A sample holder 37 is illustrated in this expanded portion 36. A further quarts tube 38 surrounding the expanded portion 36 is also arranged in the tube 8.
Inert gas may be led to. this tube 38 via a conduit 39, said gas also coming into the expanded portion 36, where-by oxygen can be excluded from the sample or test piece during a part of the heating process, when the inventive method is utilized.

With an apparatus according to Figure 2 it is possible to separate compounds, which are volatile at a given temperature, from compounds which are not volatile but which can be oxidized .at the same temperature. The filter 34 coacts

O PI to provide good mixing of the oxygen-containing gas and carbon compounds vaporized in the inert gas. Water possibly present in the test object will of course be vaporized and detected in the water detector without being oxidized.

Graphs are shown in Figure 3 indicating the result if the analysis of a metal object is carried out with oxygen-containing gas, there being contaminants of both graphite and oil on its surface. From the curve I will be seen the result of the carbon dioxide registration on the y axis as intensit (the absolute value not being denoted on the axis) , as a fun tion of time or the temperature in the first oven and, in this connection, also of the temperature increase in the first oven, this temperature being denoted on the x axis. The graph II illustrates in a similar way the result of the water detection. Two crests will clearly be seen at 300 C, caused by the oil residues, and a crest only on the carbon dioxide curve illύstratint the oxidation of graphite at abou


! Finally in Figure 4 there are shown graphs denoting the result of an analysis of a metal sample in an inert gas, there being contaminants of both graphite and oil on its surface. Graphs I and II correspond to the graphs in Figure • 3. In this case the sample is placed in the rear portion of the sample holder and is thus heated in inert gas, in this ! case argon. Since graphite is a non-volatile substance, there is no crest on graph I at 700°C, but only crests resulting from the presence of oil. In this case the crests are displaced to 00-500°C, illustrating that different mechanisms prevail for heating in atmospheres of oxygen- containing gas and inert gas.