Why would the effect of fluoride ions Protecting egg shells from acid erosion vary with different acids?

I did an experiment reacting egg shells and acid. The egg shells were soaked in varying concentrations of fluoride and it is seen the rate of reaction between egg shells and acid decreases with higher concentrations of fluoride ions.


I performed this with hydrochloric acid, citric acid and ethanoic acid. However the percentage decrease in the rate of reaction from the egg shells soaked in distilled water to the egg shells soaked in the highest concentration of fluoride ions Is calculated, it is largest for HCl, then citric and ethanoic with a 10-20% difference each time


This showed that fluoride had a greater effect for HCl than citric.


Why would this happen?


