DE1217739B - Welding point cleaning on stainless steel - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

C23g

German class: 48 al- 1/08

Number: 1217739

File number: D 39925 VI b / 48 d2

Filing date: September 26, 1962

Opening day: May 26, 1966

The invention relates to the use of an acidic, aqueous, up to 1 to 50 percent by weight of fluoride ions, 1 to 70 percent by weight of nitric acid and 1 to 6 percent by weight of acidic metal oxides, in particular Chromium trioxide solution for cleaning welds on stainless steel.

Abrasion and oxides on items made from stainless steel containing chromium are affected Contact treatment with hydrofluoric acid partially removed. A complete removal will be by forming a protective film of so-called black fire or black protection, which prevents the prevents further acid attack.

Nitric acid alone does not clean stainless steel. The addition of nitric acid to hydrofluoric acid prevents or prevents the formation of black fire or black dirt. the end for this reason mixtures of these acids have been widely used for the stated purpose.

However, these mixtures are extremely corrosive to stainless steel. A distinction is made here between two types of corrosion effects. The first is characterized by a general loss of metal, and this type is hereinafter referred to as "corrosion". The second type of corrosion is particularly susceptible to stainless steels that have been subjected to high temperature treatment. If stainless steel is subjected to high-temperature treatment, especially locally, such as during welding, a high temperature gradient is created around the heated area. In the high temperature range, the carbon and the chromium in the steel react to form chromium carbide (Cr ₃ C ₂ ). This reaction thus leads to a reduction in the chromium content in and around the heated area, thereby reducing the local passivity. The area devoid of chromium is exposed to excessive corrosive, intergranular action by a mixture of hydrofluoric acid and nitric acid, as is commonly used for cleaning stainless steels. This intergranular penetration is the type of corrosion with which the invention is primarily concerned.

When using the solution according to the invention, the intergranular attack and the corrosion significantly reduced. It is particularly advantageous to use chromium trioxide in the cleaning agent.

While a variety of stainless steel treatment baths are known, none of these treatment baths are known is suitable for cleaning welds on parts of welds on stainless steel

Applicant:

The Dow Chemical Company,

Midland, me. (V. St. A.)

Representative:

Dipl.-Ing. F. Weickmann,

Dr.-Ing. A. Weickmann,

Dipl.-Ing. H. Weickmann

and Dipl.-Phys. Dr. K. Fincke, patent attorneys,

Munich 27, Möhlstr. 22nd

Named as inventor:

Earl Fredrick Morris, Tulsa, OkIa. (V. St. A.)

Claimed priority:
V. St. v. America September 27, 1961
(140 997)

made of stainless steel to be cleaned without blackburn.

The use of acidic metal oxides, e.g. B. Chromium trioxide, as an additive to hydrofluoric acid is from the USA patent specification 2172 041 has been known for a very long time; but no one has yet recognized that one these acidic oxides to a very specific combination of acids, namely the combination of hydrofluoric acid and add nitric acid to the long-sought agent for treating To preserve welds on stainless steels, namely the means that does not prevent blackfire supplies.

The fluoride ions are supplied by hydrofluoric acid, soluble fluorides, ζ. B. ammonium bifluoride, Sodium fluoride, sodium hydrogen fluoride, potassium fluoride, potassium hydrogen fluoride or mixtures of these in question.

As acidic metal oxides, water-soluble metal oxides come into question, those in aqueous solution corresponding acids result, z. B. manganese, permanganese, iron, chromium, molybdenum, tungsten, Uranic, vanadium, niobium, tantalic acid and the salts of these acids, e.g. B. Manganate, Permanganate, Ferrate, Chromates, etc., provided they are in aqueous, acidic media are soluble.

609 570/530

Claims (1)

3 4 A simple corrosion test allows the constant temperature, constant fluoride ion optimal concentration of the acidic metal oxide concentration and constant nitric acid concentration determine. Test specimen of the steel to be cleaned with various acidic metal oxide concentrations are selected for this purpose and thus obtained from 0 to 6 percent by weight for testing purposes. the prepared to immerse them in a sample-to-sample concentration of 5 for 15 minutes 15% strength hydrochloric acid solution appeared ^e, abschrubbt with soap may be different. When the temperature and rinse with water. The test specimens are then low, there are small differences in concentration dried with acetone, weighed and as to their most favorable. Surface area measured. The test pieces are now placed in the assigned Then a cleaning mixture is prepared. io ten test baths for a certain period of time Concentration of fluoride ions and nitric acid immersed for about 6 hours, then escaped from the baths set. Samples of the solution prepared in this way were taken and rinsed off. They are then dried the acidic metal oxide is used in concentrations up to and including the determination of its weight loss Weight percent added. A sample of the bath, weighed. which does not contain acidic metal oxide is set aside as 15 The corrosion rate in each bath is in the following Blind sample provided. This is how a number of baths are calculated: Weight loss
Corrosion rate = Surface of the test body · exposure time The corrosion rates are then recorded in a dia- The invention exerts a corrosive effect under dynamic conditions against the concentration of acid which is recorded by the corrosive metal oxide. The optimal concentration effect under static conditions is similar to how it can then be read directly from the curve 25 obtained, for example in the case of an immersion treatment. not stirred bath results. The duration of contact in experiments similar to those previously described, treatment of the articles to be cleaned with it was found that at low concentrations of cleaning agent solution depends on the degree of purification than 1%, the corrosion ^to be performed ^on an acidic metal oxide, and of the temperature effect of the Solution is higher than that of no 30 using the cleaning bath. It can contain acidic metal oxide solution. Are used in increasingly immersion times of 2 to 8 hours, increasingly higher concentrations (still under Generally, immersion times of 6 hours, 1%), the corrosion effect reaches a peak, zufriedenwonach at bath temperatures of 24 to 6O ⁰ C it drops sharply and then at an optimal one. If necessary, other Belen values can also remain. 35 durations and temperatures are used For example, experiments described above were the. Temperatures of 27 to 49 ° C are common Type with stainless steel and with AISI-304 steel satisfactory. carried out using an aqueous solution The acidic cleaning agents according to the invention was, the 4% hydrofluoric acid, 25% nitric acid and act as a rule under the stripping or the various concentrations of chromic acid de- 40 oxides; so these will be replaced. The treatment lasted. At 49 ⁰ C the highest corrosion rate was found with the acidic cleaning agent ended when that at a chromic acid concentration of 0.4%. The abrasion or the oxide are either completely removed corrosion rate was then about lV ₃ times as large as, or if they are greatly relaxed as to pass through the one that caused the blank solution. With a mechanical means can be easily eliminated. Chromic acid concentration of 1%, the corrosion resistance fell to about a third of that when rinsing off the solution, i.e. in the case of the cleaning blank solution. If the concentration is increased, the treatment follows. After rinsing the chromic acid to 2%, a solution resulted, the cleaned object was dried, and the cleaning corrosion rate was only a fifth of that of the process is complete. Blank solution was. A solution containing 6% chromic acid 50 detergents according to the invention are best contained, had a corrosion rate about one tenth suitable for the treatment of stainless steel, that of the blank solution. which has a relatively high chromium content, for example An optimal concentration range for chromium-wise a content of at least 16% chromium,
acidity under these circumstances was between 1 and permanganates, chromates and dichromates 3%. Less chromic acid did not provide a significant 55 preferred within the scope of the invention.
Corrosion protection. More chromic acid was costly, it should be noted that higher temperatures in since the corrosive effect of the solution per added area is the intergranular through-concentration unit of chromic acid only slightly increase the penetration much more than the general one decreased. Corrosion. So if stainless steel is When carrying out the cleaning process 60 welded areas are to be cleaned, the objects to be cleaned can be immersed in the cleaning agent solution and the areas indicated there can be selected.
will hold until the desired cleaning effect is observed. Another possibility is a patent claim: also in it, the cleaning agent solution over the upper 65 surface of the object to be cleaned flow to the use of an acidic, aqueous, to 1 to permit. It may also be beneficial to add 50 percent by weight fluoride ions, to 1 to 70 percent cleaning agent solution stir. The cleaning agent according to weight percent nitric acid and 1 to 6 weight 5 6 weight percent acidic metal oxides, particularly U.S. Patent No. 2,172,041; Chromium trioxide, containing solution for the welding yearbook of surface technology, 1958, p. 800 and Spot cleaning on stainless steel. 809; Chemisches Zentralblatt, 1959, p. 16470 (treatises of publications considered: 5 lung by N. W. Bogojawlenskaja and German Auslegeschrift No. 1 077 503; N. W. L i ρ k i n). 609 S70 / 530 5.66 © Bundesdruckerei Berlin