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Description / Table of Contents: Corrosion of High Alloy Steels by Nitric AcidThe corrosion diagrams and iso corrosion lines of temperable and nontemperable chrome steels, chrome-nickel steels and some others containing further alloy constituents in nitric acid solutions up to 67% are determined at all temperatures up to their boiling points, under atmospheric pressure. The resistance to corrosion of the different types of steel under definite conditions can be derived from these curves.Parts of the corrosion data obtained were used to study the influence of different alloy constituents by means of a graphic method. To achieve passivatation the minimum content of chrome increases with rising concentration of nitric acid and with rising content of carbon in the alloy. Exceeding additions of chrome exert an inhibiting influence too. Manganese is very effective especially in alloys containing up to 0.7 %. Ni and Cu and Si in particular cause increasing aggression. Mo does not influence corrosion, except in boiling acid solutions of 67% where corrosion is decreases at Mo contents up to 2%. At higher contents the corrosive attack rises.Over a wide range of concentrations corrosion tests were carried out in boiling solutions of diluted acids. With chrome steels maximum aggression occurred in the range of 0.1-1 n acid solutions if the content of chrome, with deduction of the amount of chrome necessary to form chrome iron carbides, was lower than 14.5%. The corrosion peaks are usually higher than the data of the attack of a 1.0 n nitric acid. Austenitic steels do not show behaviour like this except for 18/8 Cr/Ni automate steel which contains some sulphur. Its corrosion peak is in the range of 0.1-1.0n. In higher concentrated acids it corrodes much more than the usual 18/8 Cr/Ni steels.

Description / Table of Contents: Some observations on the corrosion properties of hardenable chrome steelsThe hardenable Cr steels needed for the cutting of modern synthetic textile fibres and for the moulding of new plastics are required to possess an ever higher corrosion resistance. The corrosion tests carried out with a number of our hardenable Cr steels in condensed water, artificial sea water, NaCl-solution with H2O2 and 6 % acetic acid showed that the corrosion rate is, generally, reduced as the residual chrome content increases. This is the chrome content which remains after deduction of 13 times the C content from the analytically found Cr content. The amount of residual chrome required for the suppression of corrosion depends on the corrosive agent, and should be around 10 percent. In certain cases, other constituents, too, may have a decisive influence, e.g. Mo in the case of acetic acid.With 50 % nitric acid at 60° C, all the steels showed a very low corrosion rate whilst 2% nitric acid at boiling temperature often showed very high values and, in some cases, produced in a test of 24 hours' duration considerable inter-crystalline corrosion. It has not been possible, So far, to find fully convincing explanations for these phenomena with the aid o f micrographs, hardness measurements, or separate analyses. It was therefore decided to carry out systematic tests with heat-treated specimens in boiling 2% nitric acid. With hardening at temperatures between 980 and 1130°C, it was found that a residual chrome content below 10 per cent. caused a high degree of corrosion with intercrystalline effect, whilst a content above 10 per cent. caused weak corrosion with intercrystalline effect, and over 12 per cent. very little corrosion, without intercrystalline effect. With a residual chrome content above 12 per cent., it was no longer possible to detect any influence of the hardening temperatures. With inadequate Cr content, however, the temperature influence can become very great. The significance of the residual chrome values of 10 and 12 per cent. can also be ascertained with the aid of a statistical survey and the relevant diagram.Further tests with two steels containing more than 12 per cent. residual chrome revealed no influence of the forging temperature. When forged, the specimens are highly sensitive to corrosion, but this sensitivity disappears after annealing at 800° C. as well as afler hardening in oil at 1000° C. In contrast, tempering of the hardened specimens at 450 and 550° C results in a very high degree of general and intercrystalline corrosion proneness which is, however, no longer encountered at 650 and 750° C. Such corrosion is explained by the formation of zones with low chrome content in the vicinity of the residual austenite, due to the formation of chromium carbide, and these losses can, at low temperatures, not be compensated by chrome diffusion.To obtain the highest corrosion resistance, it is therefore necessary to ensure a residual chrome content exceeding 12 per cent. as well es a heat treatment which, in any case, avoids tempering between 300 and 600° C. The easiest way of testing the corrosion resistance is by exposing the specimen to 2 per cent. nitric acid at boiling temperature for 7 · 24 hours.

Description / Table of Contents: Nitric acid corrosion on highly alloyed steelsCorrosion tests in nitric acid up to 67 per cent. and at temperatures up to atmospheric boiling point have been carried out with steels of high chromium contents (25 to 30 per cent.) containing up to 1.64 per cent. carbon and, in some cases, also 20 per cent. nickel. The results were plotted in the form of iso-corrosion diagrams which can be used to predict the corrosion resistance was shown by a steel containing 0.1 per cent. C, 30 per cent. Cr, and 20 per cent. Ni. Even after heat treatment at 680° C, this steel does not show any intercrystalline corrosion when exposed to 50 and 67 per cent. nitric acid at boiling temperature.The results of these tests and the values for chromium steels discussed in the first part of the article were plotted, for HNO3 concentrations ranging from 30 to 67 per cent., at boiling temperature, to show the degree of corrosion on a logarithmic scale, as a function of the C and Cr contents on a decimal scale. The curves thus plotted were straight lines, i.e. the influence of carbon as well as chromium can be expressed by parabolas, rising with the concentration in the case of carbon, and falling with the concentration in the case of chromium. In the latter case, the part bonded with the carbon, which roughly corresponds to carbide (Cr3C) and amounts to 13 times the C content, must be deducted from the total chromium content.The high No content of 20 per cent. has no noticeable influence on the degree of corrosion. From a corrosion-chemical point of view, it is therefore immaterial whether the structure is of the ferrite or austenite type. The essential constituent is the free Cr content, the effect of which is governed by a parabolic law, and not by the previously much discussed n/8 Mol law.In plotting the effect of the carbon, one obtains three parabols which apply to the Cr contents of 13, 17 und 25 per cent., respectively. Below 0.1 per cent. C, the corrosion resistance is, in the main, exclusively governed by the Cr content.

Description / Table of Contents: Corrosion by Sulfuric AcidCorrosion tests over the whole range of concentration and temperature up to the atmospheric boiling point have been carried out on high alloyed Cr-Ni-Mo-Cu-steels, recommended because of their sulfuric acid resistance, and modern Mo-containing nickel alloys. The corrosion data are represented in chart form. The corrosion resistance regions are delineated by isocorrosion lines.All the Cr-Ni-Mo-Cu-steels show about the same behavior in more than 50 percent sulfuric acid solutions. In this region they are of limited use but in solutions of lower concentrations they are technically applicable even up to higher temperatures. The limits of use depend in general on their content of nickel.The high alloyed nickel alloys prove generally increased sulfuric acid resistance. In contrary to all the other tested materials the Ni-Mo-alloy 70/30 (Euzonit 70) shows its best sulfuric acid resistance in the concentration region of about 80 percent even up to relatively high temperatures. This behavior is explained as to be caused by the low content of iron. The effect of chrome in alloys is obviously very similar to that of iron.