Democracy is quite premune from corruption. In that, democracy is singular compared to the other regimes.
Reports about corrupted behaviours are scarce in democratic countries and this can only be explained by understanding what corruption means.

Corruption intervenes when there is a distortion of an expected behaviour.
Many other regimes cast strong lines for expected behaviours in many situations of life. They drew very strict portrays of the ideal figure acting in a peculiar social role.
For example, a young lad seeking self accomplishement by following the path of knighthood evolved in a well defined context with clear notions his achievements would be assessed by.
Deviations, small or large, from the stern definitions of the perfect expected behaviour could give a solid basis to characterise a corrupted behaviour.

Democracy has no other expectation from democratic people than they express their natural leanings, their human nature.
There is no such predetermined behaviour templates.
Human actions can only be understood as the expression of human traits.
There is no strict patterns democratic people could deviate from.
A knight fleeing from a battlefield could be branded as a coward.
A knight assassinating his prisoners could be branded as a treacherous felon.
Because there was a code, a code of behaviour determining human actions and behaviours.
A democratic person acting the same in a democratic context cant be branded the same just because he/she is expressing human nature traits which is the only expection democracy has towards him/her.

Yet democratic countries as very organised societies have to define social parts for the sake of the communauty. Hence expectations from people who choose to play these parts.
Again there is something explaining that corruption can barely exist in democracy.
It is one of the democratic hard core beliefs called free will.
Somebody analysing behaviours from an absolute point of view can only characterise the behaviour of a business man fixing a market under the pressure of one customer as corrupted.
Putting this back into its context brings of course different conclusions.
Take for example the case of a journalist who is receiving money, who is offered the student fees for his childrens, who is offered restaurants, travel tickets, hotel rooms, prostitutes for his pleasure, who is embedded in a military unit on whom his life depends, all that from the people he is in charge of reporting about.
Regimes not believing in free will conclude that the resulting behaviour is a corrupted behaviour and then to the existence of corruption.
In democracy, thanks to the free will notion, there is actually no belief that all these presents can have influenced the journalist in such a way he is going to report biasedly, in favour of the people he received the presents from.
The only cases that can be branded as corruption in democracy are the cases for which the two parties put out a clear and legal contract binding the two parties to act in a certain way.
If a political man and a business man put clearly in a legal contract that the political man commits himself in favouring the business man for the obtention of a public contract in exchange of a given amount of money when there should be an open competition between bidders, this is a clear cut case of corruption.
If such a contract doesnt exist, corruption cant simply exist.