Acids nickel-based alloys

Pla.tinum, Platinum plating has found appHcation in the production of platinised titanium, niobium, or tantalum anodes which are used as insoluble anodes in many other plating solutions (see Metalanodes). Plating solutions were often based on platinum "P" salt, which is diamminedinitroplatiniim (IT). A dinitroplatinite sulfate—sulfuric acid bath has been used to plate direcdy onto titanium (129). This bath contains 5 g/L of the platinum salt, pH adjusted to 2.0 with sulfuric acid. The bath is operated at 40°C at 10—100 A/m. Other baths based on chloroplatinic acid have been used in both acid and alkaline formulations the acid bath uses 20 g/L of the platinum salt and 300 g/L hydrochloric acid at 65° C and 10—200 A/m. The alkaline bath uses 10 g/L of the platinum salt, 60 g/L of ammonium phosphate and ammonium hydroxide to give a pH of 2.5—9.0. The alkaline bath can be plated directly onto nickel-base alloys acid baths require a gold strike on most metals. 

Replacing some of the nickel with iron produces a family of alltws with intermediate corrosion resistance between stainless steels and the Ni-Cr-Mo alloys. Alloys such as Incoloy 825 and Hastelloy G-3 and G-30 are in this family. Incoloy 825 has 40 percent Ni, 21 percent Cr, 3 percent Mo, and 2.25 percent Cu. Hastelloy G-3 contains 44 percent Ni, 22 percent Cr, 6.5 percent Mo, and 0.05 percent C maximum. These alloys have extensive applications in sulfuric acid systems. Because of their increased nickel and molybdenum contents they are more tolerant of chloride-ion contamination than are standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. Many of the nickel-based alloys are proprietary and are coverecf by the following specifications ... 

Takeda et al. [141] described the conditions applied for the diffusion bonding of nickel-based alloys (Hastelloy). A test piece of a plate and fin heat exchanger (dimensions 40 mm x 40 mm x 3 mm, channel dimensions 1000 pm x 1000 pm) was first cleaned with a 1% solution of nitric acid and hydrogen fluoride to remove the oxidation layer. A pressure as low as 6 mPa was necessary to run the process. For NiCrFe and other materials, other authors claim that an even lower pressure of 1 pPa is required. A welding temperature of 1 150 °C and a contact pressure of... 

The effects of acid concentration and temperature on the anodic polarization of a commercial nickel-base alloy (Hastelloy C, nominal composition 54 wt%Ni, 2.5 wt% Co, 15.5 wt% Cr, 16 wt% Mo, 4 wt% W, 5.5 wt% Fe, 0.06 wt% C maximum) are shown in Fig. 5.32 (Ref 29). Qualitative conclusions from these curves indicate that the changes in corrosion rate on increasing the acid concentration from 1 to 10 N should be relatively small but that the effect of increasing the tempera-... 

Nickel based alloys, such as Hastelloy, have outstanding corrosion resistance in reducing acids, mixed acids and acids at high temperatures. The principal restriction in their use is the high cost. 

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

The enhancement of creep by anodic dissolution is known in the case of copper in acetic acid (123) and austenitic stainless steels and nickel-based alloys in PWR environments. The initial vacancy injection from the surface is followed by vacancy attraction to the dislocations inside, which promotes easier glide, climb, and crossing of microstructural barriers. This mechanism illustrates the corrosion-enhanced plasticity approach (73). 

Sulfite pulping chemical recovery consists of greater amounts of sulfite, hydrogen sulfide, and hydrochloric acid than those used in the Kraft process. Because of these corrosive species, the internal portions of the recovery boilers and the evaporators are generally constructed of reinforced plastics, type 316L stainless steel, type 317L stainless steel, or nickel-based alloys. To prevent pitting and crevice corrosion, scale build-up should be prevented, wet-dry zones should be avoided, and chloride concentration should be kept to a minimum. 

Nickel-based alloys, e.g., Hastelloy, Inconel, Monel, Incoloy, Carpenter 20 400 (750) Aqueous nitric and organic acids, flue gases, chlorine, bromine, halogenated hydrocarbons, ammonia, sulfur dioxide, sulfur trioxide, organic solvents, brackish water and seawater... 

The most common intergranular corrosion tests are outlined in ASTM A 262 and G 28. Most of these test solutions are strong oxidizing acids or acids with added oxidizers that maintain the specimen at a stable electrochemical potential where accelerated intergranular corrosion will occur in a susceptible material. The test duration is typically 24-120 h, depending on the alloy. Recommended exposure periods for various nickel-base alloys are listed in ASTM G 28 but not in ASTM A 262. Though only austenitic stairrless steels are listed in ASTM A 262, some of the test methods are used for nickel-base alloys. It is important to note that ASTM A 262, Practice A, the cBialic add etch test, is recommended for stainless steels. It is not normally used for nickel-base alloys. 

---