Phosphinic acid

Storage stability and reactivity towards container material

Description of key information

Metal Corrosivity of Hypophosphorous Acid was assessed by tests which were carried out according to the UN Classification Procedure, Test Methods and Criteria Relating to Substances of Class 8, Test C.1. (http://live.unece.org/fileadmin/DAM/trans/danger/publi/manual/Rev5/English/03en_part3.pdf pp385-388)
Approximately 1 litre of 50% hypophosphorous acid was charged to a 2 litre jacketed glass vessel fitted with reflux condenser and heated using an external Haake oil heater/circulator. Three pre-weighed test coupons were suspended by PTFE tape such that one was fully immersed in the liquid, one half-immersed and the other suspended in the vapour space above the liquid surface. Separate tests were run for aluminium and steel coupons. The vessel was heated at 55°C for 7 days, after which the coupons were washed in water, all loose deposits removed with a plastic brush, rinsed with acetone, dried and re-weighed.
According to the results; Steel showed a maximum uniform corrosion rate of 55.22mm/year on the fully immersed coupon, which is far in excess of the 6.25mm/year threshold for classification as corrosive. Aluminium showed no uniform corrosion, but exhibited localized corrosion with a maximum intrusion depth of 140μm which exceeds the threshold of 120μm for classification as corrosive.
According to CLP the classification of the substance is to be considered as H290: May be corrosive to metals.

Additional information

The steel coupons showed a maximum corrosion rate of 55.22mm/year based on the fully immersed coupon. Vigorous effervescence from wetted surfaces was observed immediately on contact with the HPA solution. The effervescence gradually subsided and had ceased after four days immersion. All immersed surfaces were substantially corroded away by the end of the test period. The non-immersed part of the half-immersed coupon developed a remarkable “fan-like” encrustation emanating from the water line. The liquor became cloudy with a large amount of grey sediment and a strong phosphine-like odour.

The aluminium coupons also showed effervescence from wetted surfaces immediately on immersion, though less vigorous than the steel. As with steel, the effervescence gradually subsided and had ceased after four days immersion. None of the coupons exhibited weight loss, and a small gain in weight of the immersed coupons was measured. However, the immersed surfaces were blackened and exhibited localized blistering and flaking. The test protocol specifies that when corrosion is localized instead of uniform the depth of the deepest intrusion is used to determine the classification. For an immersion period of seven days the threshold is an intrusion depth of at least 120μm (0.12mm). The thickness of the coupons was measured using a micrometer placed across the corroded and non-corroded parts of each coupon.

According to the results; Steel showed a maximum uniform corrosion rate of 55.22mm/year on the fully immersed coupon, which is far in excess of the 6.25mm/year threshold for classification as corrosive. Aluminium showed no uniform corrosion, but exhibited localized corrosion with a maximum intrusion depth of 140μm which exceeds the threshold of 120μm for classification as corrosive.

According to CLP the classification of the substance is to be considered as H290: May be corrosive to metals.