Chromium tests

The sensitivity test on nickel-chromium test blocks show that products which give bad results to over washing tests generally do not give good results to sensitivity tests. 

Put the discharge lamp corresponding to the element to be tested in the light source position. For the chromium test, the lamp emits the spectral line at a wavelength of 357.9 run. 

Adjust the gas flow by computer. In the chromium test using the Perkin Elmer Lamda 4B instrument, a 6.21 min flow rate of air and a 3.9 1 min flow rate of acetylene is preferred. 

At present, the copper sulfate test with 50 % acid is specified in ASTM A 763 only for ferritic alloys with 25 % or more chromium. Tests on Type 430 (14-18 % chromium) [37] and on Type 321 (17-19 % chromium) steels [33] show that the 50 % acid test is also applicable to these alloys. Thus, Practices Z in ASTM A 763 and E in ASTM A 263 could be eliminated. All remaining copper sulfate tests (Practices F and Y) would then be in 50 % acid solution. 

Mercury salts, molybdates, and also vanadates give blue to violet compounds with diphenylcarbazide in acid solution, and therefore interfere with the chromium test. The interference can be prevented by the addition of suitable compounds which lower the ionic concentration of the interfering elements below that required for the diphenylcarbazide reaction. For mercury, it is sufficient to add an excess of hydrochloric acid or alkali chloride the usual low dissociation of mercury chloride is thus even further reduced. (The formation of the complex [HgClJ ions also helps to lower the ionic concentration of mercury.) When chromium is to be detected in the presence of mercury, hydrochloric acid is used to acidify the alkaline chromate solution. In this way, 0.25 y chromium is easily detected by a spot reaction in the presence of 2.5 mg mercury (1 10,000). 

The sensitivity tests are carried out on artificial defects (nickel-chromium specimens of NFA 09.520,see figure 3 of annex 1) and natural defects (one part in "light" alloy, one part in stellite grade 1 containing micropores, 2 specimens of fracture mechanical type CT20 in Z2 CN 12.10 (NFA 03.180). 

The tables 5 and 6 give the results of sensitivity tests obtained on the nickel-chromium specimens of 20 and 10 pm for the fluorescent range products, and of 50 and 30 pm for the colored range of products. 

Table 6 classification in terms of decreasing sensitivity performances of coloured products obtained on the Nickel-chromium specimens of 50 et 30 pm Tests realized on nickel-chomium specimens of 20pm with coloured products don t change the classification. 

For coloured product family selection is made on nickel-chromium specimen of 30 pm, but for fluorescent product family selection is made on nickel-chromium specimen of 10 pm. Tests made on real defects confirm these results. 

Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

Metabolic Functions. Chromium (ITT) potentiates the action of insulin and may be considered a cofactor for insulin (137,138). In in vitro tests of epididymal fat tissue of chromium-deficient rats, Cr(III) increases the uptake of glucose only in the presence of insulin (137). The interaction of Cr(III) and insulin also is demonstrated by experimental results indicating an effect of Cr(III) in translocation of sugars into ceUs at the first step of sugar metaboHsm. Chromium is thought to form a complex with insulin and insulin receptors (136). 

The classical wet-chemical quaUtative identification of chromium is accompHshed by the intense red-violet color that develops when aqueous Cr(VI) reacts with (5)-diphenylcarba2ide under acidic conditions (95). This test is sensitive to 0.003 ppm Cr, and the reagent is also useful for quantitative analysis of trace quantities of Cr (96). Instmmental quaUtative identification is possible using inductively coupled argon plasma—atomic emission spectroscopy... 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

The Electrolytic Corrosion Test. Also developed for use on nickel—chromium and copper—nickel—chromium decorative automobile parts is the electrolytic corrosion (EC) test (44). Plated specimens or parts are made anodic in a corrosive electrolyte under controlled conditions for 2 min, and then tested for penetration to the substrate. 

A.STM B650, Standard Specification for Engineering Chromium Coatings on Ferrous SuhstrateSs American Society for Testing and Materials, Philadelphia, Pa, 1985. 

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