Weak acid

A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution. These acids have higher pKa than strong acids, which release all of their hydrogen atoms when dissolved in water.
Examples of weak acids include acetic acid (CH₃COOH) and oxalic acid (H₂C₂O₄).

Dissociation

Weak acids ionize in water solution to only a moderate extent; that is, if the acid was represented by the general formula HA, then in aqueous solution a significant amount of undissociated HA still remains. Weak acids in water dissociate as:

The equilibrium concentrations of reactants and products are related by the acid dissociation constant expression, (K_a):

The greater the value of K_a, the more the formation of H⁺ is favored, and the lower the pH of the solution. The K_a of weak acids varies between 1.8×10^-16 and 55.5. Acids with a K_a less than 1.8×10^-16 are weaker acids than water.

Corrosivity

While strong acids are generally assumed to be the most corrosive, this is not always true. The carborane superacid (H(CHB₁₁Cl₁₁), which is one million times stronger than sulfuric acid, is entirely non-corrosive, whereas the weak acid hydrofluoric acid (HF) is extremely corrosive and can dissolve, among other things, glass and all metals except iridium.


http://en.wikipedia.org/wiki/Weak_acid