Washing soda or sodium carbonate or sal soda, like borax is a precipitating type of water softener, which is not as bad with detergents as it was with soaps, but it is a very harsh alkali which needs buffering agents to prevent corrosion of metal parts. Sodium metasilicate is one of the buffering agents used for this. Washing soda also needs to be well rinsed from the fabrics or they will feel harsh when dry.
CU did a test on wringer washers to see how resistant machines were to corrosion from the alkaline agents used to break the water before soap was added. They filled the machines with water and the alkaline compound and let them sit either overnight or for 24 hours. When they examined the machines, some tranny shafts were almost eaten through and they published pictures.
This was one of the major ingredients in Arm & Hammer laundry detergent which "contained baking soda" and hoodwinked people into believing they were washing in something gentle and phosphate free. Washing soda is not nearly as effective at handling the calcium-magnesium salts in hard water as phospates. Whether from not using enough to overcome the minerals in hard water or from insufficient rinsing, this product helped mineral up a lot of washers. Also, crystals of minerals left in fabrics lead to microabrasion that weakens the fibers and prematurely ages them, making clothes wear out faster. When people had to do laundry by hand, it was the strong alkaline products like lye soap and washing soda that caused such severe irritation to the hands and arms exposed to the washing solution as the clothes were scrubbed.