Metals hardness scales

MohsAn early (1822) hardness comparison test involved assigning a relative number to aH known materials (usuaHy minerals and pure metals) by virtue of their relative abHity to scratch one another. The results of this classification are not relatable to other properties of materials or to other measures of hardness. As a result of this limited useflilness, the Mohs hardness test is primarily used for mineral identification. Some examples of the Mohs hardness scale, which ranks materials from 1 to 10, are Hsted in Table 6. 

Barcol Indenter. The Barcol hardness tester is a hand-held, spring-loaded instmment with a steel indenter developed for use on hard plastics and soft metals (ASTM D2583) (2). In use the indenter is forced into the sample surface and a hardness number is read direcdy off the integral dial indicator caUbrated on a 0 to 100 scale. Barcol hardness numbers do not relate to nor can they be converted to other hardness scales. The Barcol instmment is caUbrated at each use by indenting an aluminum ahoy standard disk suppHed with it. The Barcol test is relatively insensitive to surface condition but may be affected by test sample size and thickness. 

The practice of corrosion inhibition requires that the inhibitive species should have easy access to the metal surface. Surfaces should therefore be clean and not contaminated by oil, grease, corrosion products, water hardness scales, etc. Furthermore, care should be taken to avoid the presence of deposited solid particles, e.g. stones, swarf, building materials, etc. This ideal state of affairs is often difficult to achieve but there are many cases where less than adequate consideration has been given to the preparation of systems to receive inhibitive treatment. Acid treatments, notably with 3-5% citric acid, with or without associated detergent washes, are often recommended and adopted for cleaning systems prior to inhibition. However, it is not always appreciated that these treatments will not remove particulate material particularly when, as is often the case, the material is insoluble in acids. 

Tests for indention under load are performed basically like the ASTM measure the hardness of other materials, such as metals and ceramics. There are at least four popular hardness scales in use. Shore A and Shore D is for soft to relatively hard plastics and elastomers. Barcol is used from the mid-range of Shore D to above it as well as RPs. Rockwell M is used for very hard plastics (Chapter 5, MECHANICAL PROPERTY, Hardness),... 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

Mohs hardness is a measure of the relative hardness and resistance to scratching between minerals. Other hardness scales rely on the ability to create an indentation into the tested mineral (such as the Rockwell, Vickers, and Brinell hardness - these are used mainly to determine hardness in metals and metal alloys). The scratch hardness is related to the breaking of the chemical bonds in the material, creation of micro fractures on the surface, or displacing atoms in the metals of the mineral. Generally, minerals with covalent bonds are the hardest while minerals with ionic, metallic, or van der Waals bonding are much softer. 

Fig. 4.5.2. Graph showing relationship of different hardness scales for minerals and metals. (After Winkler, 1973)...

Some surface water supplies have only 10 to 50 ppm or more of total calcium and magnesium hardness they are naturally soft waters and may also be described as lean waters, due to dissolved solids from all sources being limited to perhaps only 30 to 60 ppm TDS. Cooling systems using this quality of water as makeup may employ cycles of concentration (COC) of 7 to 1 Ox or more. Control of hardness scales tends not to be an onerous task for modem polymeric scale inhibitors, but lean water formulations must allow for more aggressive conditions and a real risk of some metal wastage. 

Despite efforts to comply with the limitations on feed water quality, CEDI systems can still foul and scale with microbes, organics, iron and manganese, and calcium- and silica-based scales. This usually occurs due to upsets in the pretreatment system or a deficiency in the system design that result in excursion in feed water quality to the CEDI system. Organics, metals, hardness, and silica problems are usually found on the membranes and sometimes on the resin (as is the case with organics). Biofouling is typically found on the... 

Water hardness is related to polyvalent metallic ions in water, and it is reported as an equivalent concentration of calcium carbonate (CaC03). A commonly used hardness classification is given in Table 13.9. Hardness is associated with the hard scale (metal-carbonate precipitates) forming in cooking utensils, pipes, hot water tanks, and boilers. This scale reduces the capacity of pipes to carry water and transmit heat well. 

The Mohs hardness scale has been widely used by mineralogists and lapidaries. It is not, however, well suited for metals since the intervals are not well spaced in the higher ranges of hardness and most harder metals in fact have a Mohs hardness ranging between 4 and 8. 

As the metallic hard materials, particularly the carbides, alloy easily with iron group metals, they are important in the production of cemented carbides (see Section 5.6.5.4). Apart from their great hardness, which is in the range 9 to 10 on the Mohs scale, they exhibit comparable electrical and thermal conductivities to metals, very high melting points and high chemical resistance. 

The silicides exhibit the lowest melting points and hardness values of the metallic hard materials (see Table 5.6-5). Their brittleness makes them unsuitable for utilization in hard metal alloys. Silicides have only been utilized industrially in metallurgical fields in which their scaling resistance and chemical resistance are important. They are also deposited using the CVD process e.g. as protective layers on high melting metallic surfaces. 

Rockwell Various indenter types/loads Dimensionless number and various hardness scales Widely used for metals but not often for ceramics except cemented carbides... 

The corrosion of zinc induces a more alkaline solution at the metal surface. Unless the precipitation is so rapid that the film is porous and thus nonprotective, such alkalinity can help to accelerate precipitation of a hard scale. 

Table 1.2. Hardness scales for metals and advanced ceramics...

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